The sensitivity of intrinsic fiber-optic Fabry-Perot (F-P) magnetic sensor is studied and the influence of linewidth of light source is discussed. The results show that the linewidth is narrower,the sensitivity will be higher. When the linewidth and the length of F-P cavity are chosen, there is an optimum reflectivity. If reflectivity of F-P cavity equals the optimum, the magnetic field detected is the minimum and sensitivity is the highest.