The invention discloses a combined gravity control system of a deep sea mining robot. The combined gravity control system involves a feeding mechanism, a ballast mechanism and a discharging mechanism,a feeding conveying belt and a discharging conveying belt of the feeding mechanism are arranged on a conveying roller in a sleeving mode and are carried out continuously transmission, the bottom of aballast tank of the ballast mechanism is connected with a compression rod, the bottom of the compression rod is connected with a push rod through a transmission mechanism, the tail end of the push rod is connected with a sealing plate embedded in a sealing cavity, a second-order push plate is arranged in a discharging cabin of the discharging mechanism, the sealing cavity is connected with the discharging cabin through a communicating pipe, the sealing cavity and the communicating pipe communicate with each other and are filled with sealing media, a piston is embedded in the communicating pipe, the piston is connected with the second-order push plate in the discharging cabin through a piston rod, and the sealing plate acts on the sealing medium to control the piston and the second-order pushing plate to move left and right. The combined gravity control system has the advantages that feeding and discharging can be controlled to reach a stable and balanced state, the stability and the accuracy of the system are achieved, and ballast can be effectively controlled, so that the deep sea mining robot can reach a follow-up balance adjusting state.