Abstract: When the ambient temperature is higher than the standard temperature, the inner hole of the standard ring gauge will undergo thermal deformation, which affects the accuracy of the measurement results. To study its thermal deformation characteristics, the thermal deformation law of the standard ring gauges under different sizes and constraint conditions is analyzed through 3D modeling and COMSOL simulation software. The applicability of the linear expansion heat formula in the calculation of thermal deformation of standard ring gauge inner hole diameter is studied in an unconstrained state, and the accuracy of the theory is verified through experiments. Further research has found that the thermal deformation of the inner diameter of the ring gauge is closely related to the constraint position, and different fixed positions have different effects on the results of thermal deformation. In a constrained state, the thermal deformation of the inner hole diameter of the ring gauge under local constraints reaches twice the theoretical calculation value. Then, the thermal deformation characteristics under different constraint conditions such as bottom fixation, inner ring fixation, and outer ring fixation are analyzed in detail. It is found that in some cases, non-linear trends may occur. In addition, the accuracy of the thermal deformation law of the ring gauge under constrained state is further confirmed through comparison with ANSYS simulation software. This study not only analyzes the variation of thermal deformation of standard ring gauges with size, but also provides reference for the calibration of standard ring gauges.