This article briefly describs the measurement of electron temperature in HL-1M Tokamak plasma and the way of relativistic modification due to blue shift in the case of higher electron temperature. A simple formula to calculate electron temperature is given under the condition that relativistic modification is taken into account on HL-1M Tokamak The blueshift is approximately proportional to the electron temperature. When electron temperature is hundreds of eV’ the blueshift is small, when electron temperature is thousands of eV, the blueshift is great The error of electron temperature is proportional to electron temperature to the forth power. When electron temperature is higher than 1 keV, the error is great than that of not taking into account relativistic effect, it is about 100-200 eV. When electron temperature is below 1 keV, the error is smaller than that of not taking into account relativistic effect, it is about 5-60 eV.