Abstract: There is no relevant criterion for the second circuit water quality of the high temperature gas-cooled reactor (HTGR) and for the dosing control indexes. Based on the research of the material and operating conditions of the second circuit, referring to the operation experience of PWRs and Once-through Boiler, when the pH of the second circuit water was set between 9.5~9.8 and the dosing hydrazine was controlled in 80~120 μg/L, better corrosion prevention effect can be achieved in the second circuit. Based on the design status of the dosing system for the second circuit of high temperature reactors, this paper put forward the optimization and modification scheme of dosing system design, using the hydrazine meter instead of dissolved oxygen meter, calculating pH by conductivity, thus to avoid the dissolved oxygen and pH meter lag and instability. Finally, by improving the control method, the automatic and precise dosing of ammonia and hydrazine in the second circuit of HTGR can be realized.