To study the quasi-isentropic compression of solid explosive, a graded areal density flyer termed the 'bed of nails'(BON) was used. Through changing the flying spike height combined with gun loading technology and aluminum base multiple electromagnetic particle velocity gauge technique, when the buffer layer after the flyer spike hit the buffer layer then impacted the solid explosive, the quasi-isentropic compression process generated by the front surface and inside of the explosive were tested and captured. The particle velocity at front surface and different depths of explosive inside was measured by Al-based multiple electromagnetic particle velocity gauge. The shock wave tracer was used to capture the time of shock wave reaching each depth of the explosive. The comparative experiments with different peak heights were performed. Results show that the quasi-isentropic compression duration of front surface of target are 0.37 μs for JB-9014 target and 0.22 μs for PTEF target when the flyer spike height is 2.5 mm. The front surface of the target has complete quasi-isentropic compression process and inside the target has completed the process of quasi-isentropic compression to impact. The x-t diagram of shock wave shows that the velocity of shock wave in the target is stable at the early stage, and it attenuates at later stage. When the flyer spike height and the thickness of the buffer layer are properly allocated, the quasi isentropic compression process with a wider time will take place in the target sample.