Abstract: The existing algorithm generally requires the hypothesis that the beginning time of interception signal is at the beginning of modulation cycle. To solve this problem, firstly, a novel model of intercepting STLFMCW signal was built by combining with the characteristic of continuous wave and electronic reconnaissance. Moreover, this paper proposed a detection and parameter estimation method. Based on Multi-period ambiguity function analysis of STLFMCW signal, this method utilizes Radon-Ambiguity Transform(RAT) frequency for evaluating modulation slope and the optimal order of Fractional Fourier Transform(FRFT). As a result, according to new time-frequency relationship of the interception model, the modulation cycle, modulation bandwidth and carrier frequency can be estimated by the FRFT at the optimal order. Compared with existing method, the proposed algorithm overcomes the flaw of the limit to beginning time of interception signal. As long as the intercepted signal including not less than two part of integrated positive or negative frequency modulation, the proposed method can estimate the signal parameters. The simulation results demonstrated that the proposed method keeps robust at low SNR.