A new method for passive detection problem with multi-static radar system is proposed. The direct-path signals in the surveillance channels are retained in the signal model. Utilizing both the reference signals and surveillance signals to obtain more accurate estimate for the illuminator of opportunity (IO) signals successively, a generalized likelihood ratio test (GLRT) detector is derived. Due to the extra information of the signals in the surveillance channels, the novel detector exhibits a more robust property. Numerical experiments demonstrate the robustness and high detection performance of the proposed detector.