The ability of temporal pulse reshaping is an important content in high power laser facility operation and fusion ignition experiment. In order to verify reshaping effect, the diagnostic system was optimized by optical and electrical power splitting, the system detected laser pulse shapes with two high-speed vacuum biplanar photodiodes (VPD), coupled to a four-channels 8 GHz oscilloscope. The effective dynamic range of the diagnostic system for measurement of pulses with high-power contrast was increased by splitting the signal from every VPD evenly into two channels of the oscilloscope with different sensitivity setting, the detected pulse from the four channels was stitched back together to form the complete pulse. The result show that the detector and oscilloscope combination has a 10% to 90% rise time of 60 ps, the diagnostic reaches 2 500:1 dynamic range for contrast ratios up to 100:1 at an within 2% uncertainty.