The deep sea prompt gamma ray spectrometer is a high-energy gamma ray detector which is planned to be carried by deep sea in-situ Autonomous Underwater Vehicle. The spectrometer is designed to prospect for metallic minerals using Prompt Gamma Neutron Activation Analysis. When operating at sea floor, gamma ray intensity is attenuated by sea water and it's inconvenient to supply power. As a result, a readout system with high efficiency and low power is required. This study aims to design a readout system which is able to operate in deep sea. BGO (Bismuth Germanium Oxide) crystals is used as scintillator of the detector due to its high density. And Silicon Photomultipliers (SiPMs) are used as photosensor for their small size and high gain. A readout system , including Front-End Module, Signal Conditioning Module, Data Acquisition Module and General Interface Module is designed . The system is tested with radioactive source and energy resolution of the spectrometer is 9.9%@1275keV. The readout system for deep sea prompt gamma ray spectrometer is able to operate according to instructions and the resolution meet the requirement of the project.