Abstract: When the laser generates the chaotic signal, the signal will have the periodicity because of the cavity length. That is easy to be identified and attacked when the signal is used for radar system. To solve this problem, a random sampling method is then proposed in this paper, which makes original chaotic sequence randomly sampled by different intervals. Autocorrelation and power spectrum analyses of origin and sampled chaotic sequences are discussed and analyzed. The results show that the periodicity of chaos autocorrelation and the spectrum is weakened and even disappeared; The frequency modulated waveform model based on photonic chaos is put forward for radar systems. Then the spectrum and ambiguity function are deduced and analyzed theoretically. Simulations results show that the side-lobes of sampled chaos autocorrelation are greatly lower than original chaos and power spectrum gets flatter as well. The designed radar signal not only has a sinc function spectrum but also has a thumbtack ambiguity function in accord with theoretical analysis, which proves that this signal can be used for radar system.