The multi-beam fish-finder is an acoustic equipment that uses phased array technology for long-distance, high-resolution marine biological detection. According to actual needs of the omni-directional multi-beam fish-finder in marine trawl and purse fishing,  by method of forming multiple transmitting beams in a certain sector by time-delay weighting, the field programmable gate array (FPGA) digital emission system is designed, to concentrate the emission energy for deeper and farther fish detection.The system includes a transmission control module, a DDS broadband waveform generation module, a time-shift beamforming module and a delta-sigma modulation module. Based on the existing test conditions, the beam directivity of horizontal adjacent 4-channel transmitters is tested through the laboratory pool, and the MATLAB software system is used to simulate the beam directivity and analyze the performance of modulation signal. The simulation and test results show that the digital transmitting system designed in this study has good performance, can be used to accomplish the beamforming within 30 ° in front of the array, and the error between the measured signal strength and the simulation is less than 1%.