Aiming at the low sensitivity of acceleration vector hydrophone in low frequency band, a piezoelectric single crystal hydrophone with double cantilever beam structure is designed according to the performance characteristics of lead zinc niobatelead titanate (PZNPT) piezoelectric single crystal. The finite element model is established to simulate and calculate the vibration mode and receiving performance of the vector hydrophone. The effects of several main material parameters and size parameters on the sensitivity of hydrophones to free field are analyzed emphatically. The theoretical calculation shows that the sensitivity of the vector hydrophone using piezoelectric single crystal PZNPT can reach -190 dB@250 Hz after optimization, which is more than 15 dB higher than that of the hydrophone based on piezoelectric ceramic PZT5A.