Light management in the Si bottom cell of a GaAs/Si tandem system is crucial due to the bandgap mismatch of the two materials, which results in a low current of the bottom cell. To evaluate the light coupling and light trapping, we developed an optical model to simulate the light absorption in the silicon bottom cell. This optical model is an extension of Basore's analytical model. By comparing the simulation with the measurement of the prototype tandem solar cell, we find that the Si bottom cell can gain up to 2.4 mA/cm2 photocurrent by improving light coupling. In addition, we study the impact of the rear surface reflectance on the photocurrent in the Si bottom cell. We observed that ~17% relative increase in current generated in this bottom cell can be achieve by changing the rear surface design from a full area metal contact to a local-back-surface-field configuration.