The invention discloses a combined energy system for underwater power supply. The combined energy system comprises a magnesium seawater fuel cell, a control unit, a primary lithium battery, a voltageconverter, a deep sea lander, a boosted circuit, a buck circuit, a charger, a secondary lithium battery, a protection circuit and a deep sea submersible. In the working state of the system, on one hand, the primary lithium battery and the magnesium seawater fuel cell work cooperatively to provide stable electric energy for the deep sea lander; on the other hand, the primary lithium battery supplies power to the deep sea submersible; meanwhile, the secondary lithium battery is charged; enough electric energy required by long-time detection operation of the deep sea submersible is ensured; therefore, the system can meet the requirements of long-time detection operation of deep-sea equipment, has the advantages of being stable in work, free of maintenance, low in cost, high in reliability andthe like, achieves stable power supply of deep-sea submersible electric equipment, and meanwhile solves the problem that power loss is large due to the fact that an underwater mooring line is long.