An intermediate medium heat exchanging device for a supercritical water oxidation system includes a material main loop, an intermediate medium loop and a replenishment branch. A material pump, a preheater, a spray desuperheater, a reactor and a regenerator are connected in sequence to form the material main loop; a buffer tank, a circulating pump, the regenerator, the preheater and a water cooler form the intermediate medium loop. A back pressure valve is located above the buffer tank. The replenishment branch includes a cooling water pump, an outlet of the cooling water pump is divided into two sub-branches, one sub-branch is connected with the spray desuperheater, and another sub-branch is connected with the buffer tank. The intermediate medium heat exchanging device is reasonable in structural design, and is able to maximumlly reduce the investment cost and ensure the stable operation of the system.