Abstract: The reception sensitivity of hydrophones is a crucial indicator of their acoustic performance. The advent of low-frequency, large-scale vector hydrophones presents new challenges: conventional standing wave tubes cannot calibrate these devices due to size constraints, and anechoic water tanks, with their boundary conditions, fail to separate direct and reflected waves of low-frequency signals using pulse sound technology. Addressing these issues, this paper investigates a method for free field sensitivity measurement of vector hydrophones using transient signals in small water tanks. Based on the analytical solution of surface vibration displacement of a single resonant piezoelectric transmitter under sinusoidal pulse signal excitation, the relationship between transient and steady-state signal amplitudes is analyzed. Finite element software is used for modal, harmonic response, and transient analysis, considering commonly used transmitters in hydroacoustic testing. This simulation studies the choice of sound source and receiver location for vector hydrophone sensitivity testing using transient signals. An experimental setup in a small water tank measures the hydrophone's acoustic pressure sensitivity using a comparison method. This approach overcomes spatial limitations and extends the lower frequency measurement limit for vector hydrophones in confined spaces.