In this paper, the superconducting weaklink and the influence of the feed back radiation on its I-V curve is studied, based on the constant source RSJ model. The analytical solution has been obtained and the relationship between the change of the I-V curve and the feedback radiation field intensity from the cavity has been given for the case while the field is very weak. The theoretical results show that the feedback radiation can change the amplitude and shape of V(H), but its period is unchanged.