Thermophotovoltaic systems perform the characteristics of high efficiency, high power intensity and wide range of heat sources. To further enhance its energy conversion performance, the physical mechanism of opt-electrical conversion and its influence factors need to be analyzed in detail. Therefore, based on the spectral opt-electrical conversion process in the thermophotovoltaic cell, the microscopic carrier transport model in the cell is established. The coupled opt-electrical model is used for the simulation of GaSb thermophotovoltaic cell to explore the effects of spectral characteristics and power intensity of the incident radiation on the cell performance. The results show that under the fixed incident radiation power intensity, the energy conversion efficiency of the cell shows obvious non-monotonicity with the change of wavelength and the peak appears at 1.42 μm. In addition, under the fixed wavelength, with the rise of radiation power intensity, the maximum electrical output power intensity accordingly increases while the rise of the energy conversion efficiency gradually slows down. At 1.42 μm, the rise of the energy conversion efficiency reaches the maximum about 4.85%.