Through numerical solution of the Schrodinger equation we illustrate the effects of frequency modulation laser upon the excitation of a four-level system irradiated by simultaneous laser pulse. When the laser power is sufficiently great and the frequency modulation parameters are suitable, it shows that the frequency modulation excitation applied to a Gatlssin statistical distribution of detunings can obtain more effictive three-step photoionizatioa than monochromatic or amplitude modulation excitation does.