Light trapping has been considered as an important strategy to increase the conversion efficiency of silicon thin film solar cell. It shows that photonic crystal with feature size comparable to the wavelength, for example, the silicon nanowire array has a great potential to exceed the conventional Yablonovitch 4n2 limit. Silicon nanowire array has been designed and constructed on silicon thin film solar cell due to its excellent optical properties. Generally, silicon nanowire array is used as the antireflection coating, axial or radial p-n junction of solar cell. Different applications of the silicon nanowire arrays need different optical properties. Theoretical investigations show that the optical property is strongly dependent on the structural parameters. In this work, several structural parameters including period (P), diameter (D), height (H), and filling ratio (FR) are optimized when silicon nanowire array plays different roles. Here, by using the finite difference time domain (FDTD) method, we focus on the relations between the structural parameters and the optical properties including reflection and absorption from 300 to 1100 nm. In the FDTD simulation model, the substrate material is crystal silicon film, and the silicon nanowire array is on the surface of the substrate. In this calculation, the top and the bottom of the unit cell are air with perfectly matched layers, and with periodic boundary conditions at the side walls. When the silicon nanowire array is used as the antireflection coating, the silicon nanowire array shows a lowest reflection (7.9%) with H=1.5 m, P=300 nm, and FR=0.282. When silicon nanowire array acts as axial p-n junction solar cell (the p-n junction is formed by substrate and nanowire array), the absorption efficiency reaches a maximum value of 22.3% with H=1.5 m, P=500 nm, and FR=0.55. When the silicon nanowire array acts as the radial p-n junction solar cell, the absorption efficiency could obtain a maximum value of 32.4% with H=6 m, P=300 nm, FR=0.349. In addition, the optical properties of silicon nanowire array with random diameter and position are also analyzed here. The absorption efficiency of optimized random silicon nanowire array reaches 27.8% compared with a value of 19.9% from ordered silicon nanowire array. All of these results presented here can provide a theoretical support for the silicon thin film solar cell to increase the efficiency in the future application.