Based on extended Kalman filter(EKF), a scaling factor(SF) identification method for strap-down seeker(SS) was proposed. First, a nonlinear model for SS system was set up under proportional guidance(PNG) law. Then, the extended Kalman equations were deduced on this nonlinear model, and it should be linearized at the value of EKF estimated based on Taylor expansion. At last, actions of this method on how to keep guidance system stability were studied under above conditions. According to mathematic simulation results, in this way, the SSSF could be estimated accurately and quickly, and the tolerable SSSF error scope of guidance system stability is extended, the robustness of system is promoted.