Abstract: In order to explore the characteristics of explosive's initiation and the detonation growth in the explosive under quasi-isentropic loadings, a one-dimensional initiation response test system under quasi-isentropic loadings was established by the magnetic compression device CQ-4 and the multichannel photonic Doppler velocimetry (PDV). Surfaces of five PBXC03 samples with different thicknesses were compressed by the quasi-isentropic loading so as to obtain particle velocities of sample's reverse sides. Results show that the waveforms of particle velocities obtained are clean and details of them are clear. After the quasi-isentropic loading enters into the explosive, a precursor shock wave is formed by the chasing and convergence of quasi-isentropic waves. And, initiation mechanisms of the explosive under quasi-isentropic loadings have obvious differences from that under impact shock loadings. Before the formation of the strong precursor shock wave, few hot-spots are formed and no reaction occurs in the explosive. Otherwise, the response of the explosive performs as characters of initiation and detonation dominated by the hot-spot initiation mechanism after the strong shock wave is formed.