Abstract: The existing measurement methods are difficult for measuring the directivity of the transducer with decrease of its resonance frequency and increase of its size, moreover, the transducer performance changes before and after installation. In view of this problem, a ship-board calibration method of transducer directivity in open water based on 3D hydrophone array is proposed in this paper. The positions and orientations of the transducer to be calibrated and the hydrophones array are determined by using ultra-short baseline (USBL) positioning and coordinate conversion, and the directivity diagram of the transducer is drawn by extracting the received sound pressure data of hydrophone array. Both standard measurement method and ship-board calibration method are used to measure the directivity of the transducer respectively in lake trial, and the experimental data at 5 kHz and 2.5 kHz are compared and analyzed. The result shows that the -3 dB beam-width errors in the directivity diagrams at the two frequencies measured by the two methods are 4.2% and 7.4% respectively, which meet the requirement of underwater acoustic measurement (i.e. less than 10%), so the feasibility and effectiveness of the ship-board calibration method are confirmed.