Multijunction solar cells have been demonstrated as an efficient approach to overcome the Shockley–Queisser limit and achieve high efficiency. However, cost reduction and further development of high-efficiency tandem solar cells require many device optimization studies and ask for heavy theoretical assistance. In this paper, we present a versatile alternative via an updated version of a free solar cell simulation software, wxAMPS, which incorporates a nonlocal band-to-band tunneling model and several specific physical mechanisms into the solver to better describe the device behavior of tandem solar cells. The simulation results are compared against a commercial technology computer-aided design program, showing satisfactory agreement. Moreover, a useful feature, subcell analysis, has been developed to give better insight into the individual subcell performance of the devices and to provide guidance for current matching. The algorithm used in the subcell analysis, which avoids the complexity in treating nonlocal behaviors of tunneling junctions, notably accelerates the tandem solar cell simulation, and thus allows implementing batch simulation for device optimization. The results of the subcell analysis have been used to experimentally optimize subcell thicknesses and successfully improved the efficiency of the tandem solar cell.