Offshore discarded platform piles removal wire saw is new underwater work equipment which is developed in recent years. Hydraulic system provides power for it. The reliable hydraulic system is protection for wire saw safely and stable works. The wire saw hydraulic system was designed, which hydraulic system reliability theory was analyzed, and the discarded platform piles removal wire saw hydraulic system logic diagram was obtained. According to the practical underwater wire saw specific working environment and conditions, the hydraulic system reliability was studied and calculated, which lays a foundation for the wire saw security and stable works. Experiments show that wire saw hydraulic system reliability meet the requirements of design.