Abstract: A noncollinear third-order autocorrelator was designed and manufactured for contrast ratio measurement of single shot picosecond pulses. Under the premise of obtaining correct cross-correlation signals during the parameters optimization, it was still important to consider the conversion efficiency of the third harmonic wave. The noncollinear phase matching value with the maximum efficient nonlinear coefficient was taken into account according to the polarization tensor of a nonlinear crystal KDP. The noncollinear phase matching and group-velocity mismatch were synchronously satisfied, and large noncollinear angles of the incident pulses were selected for the increase of the test range of time. The prepulse and main peak were separately measured, and two-image reconstructive technology was