Abstract: Nonlinearity is the main characteristic of thermoelectric laser power detectors, which represents the change in the detector response under different power, and has an important impact on the measurement accuracy. Based on the electrical substitution method, this paper studies the measurement method for response nonlinearity of laser power detectors at several watts to several hundred watts. An electrical calibration device was developed that can automatically measure electrical calibration responsivity under different power levels. Different from the common constant-voltage mode or constant-current mode, the device has a constant-power output mode, which can significantly improve the stability of loading power for electric calibration heaters with a large temperature coefficient. In the study, two electrical calibration laser power detectors were fabricated based on two kinds of ceramic heaters, and the response nonlinearity of the electrical calibration was measured. The results showed that for the MCH ceramic heater with a large temperature coefficient, the loading power instability under the constant-power mode was 0.025%, which is much lower than 2.0%, i.e. the instability under the constant-voltage mode. Due to the higher power stability and the automated measurement process, the evaluated uncertainty of the measured electric calibration responsivity was better than 0.10% in the power range 0.49 W to 189 W, which was an improvement on the accuracy of the nonlinearity measurement of electrical calibration.