High laser pulse contrast ratio is one of the most important terms during the interaction of ultrashort laser pulse and substance. In order to enlarge the range of the pulse contrast measurement, a third-order correlator based on the technique of cross-correlation between the fundamental and second harmonic waves of ultrashort lasers is built. Using this correlator, the contrast ratio of ultrashort laser pulses from a typical Ti:sapphire oscillator is measured. The results show that the conversion efficiency of second harmonic and third harmonic of ultrashort laser pulse can be enhanced, and the dynamic range in pulse contrast measurement can be made much wider by using convex lenses, and the correlator’s widest dynamic range in pulse contrast measurement is about 108.