A third-order correlator is built based on the technique of cross-correlation between the fundamental and second harmonic waves of femtosecond lasers. Using this correlator, the contrast ratio of femtosecond (fs) laser pulses from a typical Ti:Sapphire laser is measured. The measurements show detail features of the fs laser pulses. This also provide an effective way to meansure the contrast ratio of amplified fs laser pulses.