The writing process of photorefractive dynamic gratings in InP∶Fe was described with nonlinear carrier transport equations based on a one center-two band model, which were then linearized by perturbation. As a result, the steady-state space charge field in a small-modulation interference pattern was solved, and the relationship between the space charge field and the gain coefficient was established by coupled wave equations. Furthermore, the effects of temperature, pump intensity, external direct current (DC) electric field, and incident angle on the gain coefficient were examined. It turns out that the gain coefficient exhibits temperature-intensity resonance and the optimal pump intensity is strongly dependent on the operating temperature of crystals. The gain coefficient can be significantly improved by an external DC electric field and the gain increases linearly with the enhancement of the electric field below 10 kV·cm ^-1. There is an optimal incident angle in the case of temperature-intensity resonance. The optimal pump intensity is 218 mW·cm ^-2 and the optimal incident angle is 5° at the temperature of 298 K and the external electric field of 5 kV·cm ^-1. Also, we experimentally investigated the relationships of the pump intensity, external DC electric field, incident angle with the gain coefficient through two-wave mixing in InP∶Fe. The results show that the change rule of the gain coefficient agrees with the theoretical predictions, verifying the rationality of the one center-two band model. This work has an important reference value for the research on the two-wave mixing of III-V semiconductor photorefractive crystals.