Strain gage has been widely used in investigating the electromechanical properties of YBa₂Cu₃O_7-x(YBCO) superconducting tapes and quench detection system of superconducting devices because of its high sensitivity and simple operation. The electric current and strong adhesive are essential for the strain measurement. In order to obtain accurate strain results, the strain gage should be firmly attached to the YBCO tape, and electrified during the measurement process. Therefore, the strengthening effect and thermal effect on YBCO tape induced by strain gage cannot be avoided at low temperatures. In this paper, firstly, the strengthening effect induced by strain gage was studied experimentally and quantitatively in the uniaxial tensile process of YBCO tape. By analyzing the Young's modulus obtained by virtual extensometer and strain gage, it can be found that the Young's modulus obtained by strain gage is significantly larger, and the deviation is more than 10% compared with the result based on virtual extensometer. Then, the thermal effect was investigated in the 77 K and lower temperature environment. Through the real-time monitor by a temperature sensor, the temperature rise is found at the position of strain gage on the tape. The temperature rise of the copper-coated strip is small, which is 1.9 K maximum, while the temperature rise of the silver-coated strip (without a copper stable layer) is more than 22.1 K. At the same time, it is found that the lower the strip temperature is, the faster the temperature change caused by the strain gage. In the experimental measurement and application of YBCO tapes, the strengthening effect and thermal effect of strain gages not only have a significant impact on the experimental results, but also become a potential source of thermal runaway in industrial applications.