The invention discloses a heat loss measuring device of system thermal engineering experimental equipment and a method thereof. The method includes: step 1, determining a device of the system thermalengineering experimental equipment and covering the outer side of the device with thermal insulation cotton; step 2, injecting one or more components-containing working media into the device, and recording mass mf of an individual component and specific heat capacity cp of fluid; step 3, turning off an upper isolation valve and a lower isolation valve, and conducting loop isolation on the device to form an area to be measured; step 4, heating the device in the area to be measured, stopping heating when the temperature of the device reaches a preset temperature, and then timing; step 5, conducting real-time measurement, storing a temperature Tso at an outer wall temperature measurement point, a temperature Tsi at an inner wall temperature measurement point and a temperature Tf of the fluidin the device, and stopping the measurement when the temperature of the device is reduced to the lower limit temperature; step 6: using a data processing system to obtain solid wall mass ms, solid wall specific heat capacity cps, a temperature Tsi at the outer wall temperature measurement point, and a temperature Tso at an inner wall temperature measurement point from the area to be measured.