Distorted current causes the electronic trip unit malfunction, which results in low-voltage circuit breaker’s tripping without fault. To analyze the effect of two typical distorted waveforms, triangular wave and square wave, on the electronic trip unit with different current detection and control methods, the peak coefficient KP and waveform coefficient KW are used. The defect that total harmonic distortion (THD) can’t be used to analyze the harmonic current impact on the electronic trip unit’s calibration error correctly and quantitatively is overcome. The calibration error can be seen accurately and intuitively through the calibration error curve of peak value detection and mean value detection. According to the nonlinear load’s waveform characteristic and calibration error curve, the frequent malfunction of electronic trip unit caused by distorted current can be avoided in a certain extent by adjusting the action current setting value properly.