The invention discloses a hydrochemical feedback measurement and control system of a supercritical autoclave and a control method of the hydrochemical feedback measurement and control system and solves the problem that no equipment can be used for controlling and monitoring the water quality of a reactor under the condition of supercritical water with the temperature of 650 DEG C and the pressureof 25MPa. The hydrochemical feedback measurement and control system comprises a water tank communicating with a supercritical autoclave loop, a water inlet and a water outlet are formed in the water tank, the water inlet communicates with the supercritical autoclave loop by a water inlet pipeline, and the water outlet communicates with the supercritical autoclave loop by a water outlet pipeline; the water outlet pipeline is further provided with a detecting branch communicating with the water inlet pipeline, and the detecting branch is provided with a detecting instrument; the water inlet pipeline is provided with a chemical agent storage device communicating with the water inlet pipeline by a control switch; and the control switch is connected with a controller for controlling the on andoff of the control switch by a signal detected by the detecting instrument. The hydrochemical feedback measurement and control system has the advantages that the water quality condition of the reactoris better simulated, and the quality level of a corrosion experiment of the supercritical autoclave is guaranteed.