The signal-to-noise-retio (SNR) measurement is studied to achieve high dynamic range in pulse SNR measurement and meet the requirement of fast ignition of laser. The second order autocorrelation signal of short pulse occurring in nonlinear crystal is studied, which indicatesd that the pulse contrast of 2nd order autocorrelation equals that of original pulse. And the effect of the angle variation between two input beams on their output signal in non-collinear autocorrelation is discussed. A system of second order autocorrelation was built up to study the time characteristic of short laser pulse. Its dynamic range achieves ～108 in pulse contrast measurement.