The fractional quantum Hall effect (FQHE) arises when electrons near zero temperature confined in a 2D plane are subjected to a strong perpendicular magnetic field. Since its first discovery [1], the FQHE has always been one of the most important topics in condensed matter physics. The origin of the FQHE can be understood by the composite fermion (CF) theory, according to which emergent particles called composite fermions are formed when an even number of vortices are attached to each electron. Because the vortices partially cancel the Aharonov-Bohm phase generated by the external magnetic field, CFs experience a reduced effective magnetic field, in which Landau-level-like energy bands called the [lambda] levels are formed and filled by CFs at an integer filling. The FQHE of electrons of filling factor [nu] = n 2pn±1 is therefore mapped into the integer quantum Hall effect (IQHE) of CFs. The CF theory successfully predicts many properties of the FQHE, such as filling fractions, collective excitations, spin structures, emergent Fermi sea of CFs with well-defined Fermi wave vectors, and many more. While many great achievements have been made, lots of questions remain to be answered. For example, a typical simplification in FQHE problems is that the electron system is treated as a strict 2D system. While this approximation has been proved to be useful in many cases, it turns out that there are exceptions. In experiments, electrons are usually confined within finite quantum wells. The finite width modifies the effective interaction between electrons. It also changes the nature of the ground state by including the new degree of freedom, as the finite well allows the mixing between different subbands. Another factor of importance is called the Landau level mixing, which is usually neglected in theoretical studies under the approximation that the magnetic field is strong enough to quench electrons to the lowest Landau level. However, under typical experimental conditions at present, the magnetic field is usually not that strong, and higher Landau level components are likely to mix into the system's ground state. The Landau level mixing brings a difference in the effective interaction between electrons, and it also introduces the three-body interaction, which breaks the particle-hole symmetry. The finite width effect and the Landau level mixing effect modify the effective interaction between electrons and may lead to new phases. For example, as the repulsion between electrons is reduced due to the finite width, it is possible that the vortices attached to electrons overscreen the repulsion and make the net interaction between composite fermions attractive. The attraction, therefore, can cause the pairing of composite fermions and lead to the so-called Moore-Read Pfaffian state or its particle-hole conjugation, the anti-Pfaffian state. To quantitatively describe the influence of the finite width and the iii Landau level mixing, we develop the three-dimensional fixed-phase diffusion Monte Carlo method. This method takes care of the finite width effect and the Landau level mixing effect in a single framework, and it is not a perturbative method, which makes it suitable for studying strongly-correlated systems. Equipped with the fixed-phase diffusion Monte Carlo method, especially its 3D version, we systematically study several different systems in this thesis. We find that the finite width together with the Landau level mixing effect can lead to new phases as well as affect the systems' quantitative properties, such as the transport gap. To be explicit, we find that the FQHE at filling factor 1/2 in finite GaAs quantum wells might be the Moore-Read Pfaffian state. We also find that the charge-imbalance in such quantum wells does not favor the Moore-Read Pfaffian state; we find that the Bloch ferromagnetism of composite fermions observed in Ref. [2] might be induced by the change of the Landau level mixing; we find that the Landau level mixing and the finite width cannot fully explain the discrepancy between the theoretical calculation and the experimental measurement of the transport gaps of the FQHEs in the sequence of n 2n+1.