A new shock initiation measuring system is developed with the reverse-impact method， in which the explosive sample was driven by a gas gun and impacted the LiF window at a certain speed. The photonic Doppler velocimetry （PDV） was used to measure the interfacial particle velocity profile between the explosive sample and the LiF window. This measuring system has higher accuracy （3%）， higher time resolution （5 ns） and lower requirement of explosive samples than previous shock initiation measuring methods. Moreover， to explore the effect of aluminum powder， the shock initiation characteristics of three RDX-based aluminized explosives （RDX/Al） with different contents of aluminum powder （0， 15% and 30%） were investigated by this measuring system. The experimental data indicates that with the same impacting speed， the shock initiation reaction growth becomes slower with a higher content of aluminum powder. The interfacial particle velocity of the RDX-based aluminized explosive with 30% aluminum powder requires more time to arrive at a peak value， which is 47% longer than that of the RDX explosive. The aluminum powder makes the shock sensitivity significantly decrease and it plays a dilute role in explosive energy during the shock initiation process of aluminized explosives.