Abstract: The underwater resonance frequency function and electrical edmittence curve of slot piezoelectric ring are studied by energy principle. By establishing the equivalent ciruit diagram of slot piezoelectric ring in low frequency receiving condition, the low frequency open circuit voltage sensitivity of slot piezoelectric ring is deduced. Compared the sensitivity between slot ring and monolithic piezoelectric ring, the thought of building deep ocean hydrophone by slot ring structure and free flooded structure is presented. By establishing finite element simulation model of free flooded slot piezoelectric ring, the relationship between ring structure size and low frequency open circuit voltage sensitivity are discussed. The slot piezoelectric ring deep ocean hydrophone is researched by finite element analyze and optimized where the resonance frequency is 600 Hz. The acoustic and pressure testing indicated that the max low frequency open circuit voltage sensitivity of free flooded slot piezoelectric ring hydrophone in work bandwidth 100~300 Hz is-193.2 dB and the least value is-197.9 dB long with the-4.7 dB fluctuation. The tolerance hydrostatic pressure capability is 30 MPa. Compare with the same structure size free flooded piezoelectric ring hydrophne, the low frequency open ciruit voltage sensitivity of slot piezoelectric ring hydrophone raised 20 dB. The result verify the practicability of free flooded slot piezoelectric ring structure in deep ocean hydrophone.