According to the structure of phase locked loop, a new linear since feedback FM system is proposed,and its dynamic behaviors are studied. Based on the analysis of mathematical model of linear system since FM, the dynamic behavior can be described by using simple sine mapping. The dynamic behaviors and chaos phenomena of sine mapping are studied through phase space trajectory,bifurcation and Lyapunov exponent. It is discovered that the system bears the characteristics of periodic and symmetry on its dynamic behaviors. The frequency spectra under different conditions are compared for the chaos system of since FM,which may provide theoretical basis and numerical reference for the design of the self-FM chaos signal source.