The strain gauge is installed on the freely retractable sample, and the sample is not affected by external force. When the ambient temperature changes, the indicated strain of the strain gauge will change with the change of temperature. The change of indicating strain is partly caused by the thermal expansion and cold contraction of the sample (called the temperature strain of the sample). The other part is caused by the change of resistance of resistance wire directly caused by the change of environmental temperature. Considering the temperature compensation, because the compensation block is made of the same material as the sample, the strain change caused by the change of ambient temperature (non-external force) is the same, and the resistance measured by one arm and a half bridge is the difference between the strains of two legs, which just eliminates the influence of environmental temperature change.