Based on the cascaded frequency-harmonic mechanism during the course of nonlinear compression of nonchirped laser pulse, the sensitive factor affecting the pulse compression of crystal phase mismatch is analyzed theoretically and validated experimentally. After the input fundamental pulse (namely, 1ω, 0.8-μm wavelength and 278-fs pulse width) propagates through the type I second harmonic generation β-BaB2O4(BBO) crystal, the 1ω output pulse width is shortened firstly and widened again with the increase of crystal phase mismatch. This trend can be summarized as the “U” grapheme nonlinear change. The output pulse width is shortened to the least value and the highest compression ratio of pulse width is obtained when the crystal mismatch is 40 mm-1.