Ultra-thin silicon solar cell is a kind of good cost-effective energy solution. Moreover, nanostructured front surface and all-back contact design have been proposed as the promising methods to make ultra-thin silicon an effective solar irradiance absorber. Different nanostructures result in the different generation rate distributions and surface recombination, even though their corresponding integrated generation rates are the same. In addition, under the effect of carrier lifetime, the different generation rate distributions determine the number of carriers that reach the electrodes. Therefore, the grating nanostructures with three different section shapes are chosen for the investigation of the coupling effects of the different generation rate distributions and recombination caused by nanostructures. After consideration of the effect of the surface enhancement ratio of nanostructure, the deviation of electrical characteristics (fill factor), which is caused by the different generation rate distributions caused by nanostructures with the same integrated generation rate, is 0.20% at most as the effective surface recombination increases. Meanwhile, as the carrier lifetime decreases, the deviation of electrical characteristics (short circuit current) is only 1.80% at maximum and does not change with carrier lifetime. Therefore, it is found that, in all-back-contact ultra-thin silicon solar cell, the different generation rate distributions caused by nanostructures with the same integrated generation rate have little effect on the electrical characteristics, no matter how the effective surface recombination and carrier lifetime change. Based on the conclusion, a simplified method is developed for the electrical characteristics simulations. As the effective surface recombination and carrier lifetime change, the minimum error of electrical characteristics calculated by the simplified method is 0.06%, and the maximum is just 3.10%, which proves its accuracy.