To study the cracking phenomenon at the end of Al/steel clad pipe by interior explosive expansion under water and interface bonding form, a reliable polyvinyliene fluoride(PVDF) piezoelectric film sensor was first used for testing the impact pressure of Al/Steel coaxial pipe by interior explosive expansion under water, and the pressure history curves were obtained and compared. Results show that the impact pressure peak of base pipe and flyer pipe along the detonation wave transmission direction of metal detonating cord reveals an increasing trend, indicating that the direct cause of the cracking phenomenon at the end is an increase in pressure. The calculated mean of peak pressure is 532.9 MPa. According to the relationship between the impact velocity and the impact pressure of explosive cladding, the impact velocity obtained in the case of the known impact pressure is 52.57 m·s^-1. From the explosive welding interface bonding mechanism and the metallography knowledge, the impact temperature produced by the interface impact is not high enough to make the interface melt. The metallographic photos of interface reveals that its way of combination is direct bonding between metal without the transition layer and melting phenomenon, showing that measuring the impact pressure of interior explosive expansion under water by PVDF piezoelectric film sensor and inferring the interface bonding form by the obtained data are feasible.