Using the effect of the noncollinear phase matching seconds harmonic in a nonlinear crystal(KDP),the function of noncollinear angle as phase matching angles,temperature and wavelength are calculated theoretically,the influence of group-velocity mismatch and group-velocity dispersion in a nonlinear optical crystal are considered,then the beam diameter,the incident angles of the fundamental pulses,phase matching angles and size of crystal are determined.We developed a noncollinear second order autocorrelatior to measuring the width of singleshot laser picosecond pulses,and the pulse duration is measured.