Using the analysis model of the phase noise, the random noise of semiconductor laser is studied. The modulation on the phase and complex amplitude of laser is discussed. A beat note experiment system with two different kind of laser is established to get the noise signal. In the system, a tunable semiconductor laser and a fiber laser are employed. Since the linewidth of fiber laser is much narrower than that of the semiconductor laser, the beat note can show the noise character of the semiconductor laser. According to the statistic character of the laser phase noise, a kind of phase lock technique is introduced. The feasibility of such scheme is studied in detail.