Based on quantum theory of light, the noise power density spectrum and its time-spectrum formula of the intensity fluctuation of a two-mode laser are derived. Quantum statistical properties of a two-mode laser and its time-spectrum characteristics as well as the frequency tuning features of the frequency beat noise are analyzed and discussed. The study shows that the time-spectrum characteristics of the frequency beat noise of a two-mode laser can be applied to the frequency and power stabilization of two-mode laser can be applied to the frequency and power stabilization of two-mode lasers.