For the requirements of the measurement of the high-contrast shaped pulse Inertial Confinement Fusion(ICF) based on the cascaded photodetection of multiple pulses, experimental research on high-contrast measurement for ns high-power laser pulse is developed at SG-III laser system. Using the ultra-fast photodetector, the bottom of a concave pulse is measured through multi-channels in experiment. The experimental results show that the concave pulse obtained by the multi-pulses joint is of the contrast of 350:1 better than 30:1 obtained through single channel.