Molten salt frozen-wall is proposed as an option for protecting the metallic walls from corrosion by a layer of frozen salt which can prevent gas and liquid medium contacting with metallic wall. Accurate monitoring and controlling the thickness of frozen wall is the key for its successful application. Different on-line thickness detection methods were studied in the condition of frozen-wall forming and maintaining,and results were compared and analyzed with the thickness measured by mechanical caliper. For temperature gradient calculation method the result is more accurately in a state of balance than that in the unsteady condition,but it need to layout applicable thermocouples. The thickness of frozen wall also could be detected by cooling heat calculation method,which using outside heat exchange amount and metal wall temperature,its advantage is that there is no need for additional equipment,but its stability and accuracy depends on the amount of cooling heat,Online image can reflect thickness of frozen wall visual,but it need to be equipped with stable and reliable high temperature camera system. Because molten salt frozen-wall will be used in airtight condition which there is no water and oxygen,these on-line methods should be further researched and evaluated in the real system.