The principle and uncertainty of a low impendence standard calibrator that produces full lighting impulse is described. The calibrator is a small MAX generator, The peak value Up , front timeT1, and time to half value T2of the generated impulse are calculated from the calibrated values of the components of the impulse forming network. The operating voltage range of the calibrator is from 10V to1000 V. The expand uncertainty of Up, T1, T2 is 5×10-4, 4×10-3 , 4×10-3 respectively at 95% confidence level with a coverage factor of 2. The deviation for the peak value of measured value and set value is from -0.5‰ to 0.2‰, for front timeT1 from -1.5‰ to 2.5‰ and for time to half value T2 from -0.6‰ to 0.6‰. The accuracy for the impulse peak value is ±1‰ and for the time parameters, less than ±1%. Repeated 20 times, the max standard deviation is 2.7×10-4. During six months the long term stability is less than 0.2‰. The load is 7.95kΩ compared to 1MΩ, the differentials of measured value for Up、T1、T2 isagreement with of calculated value. The experimental results show that the low impedance calibrator is not only can be used as the primary reference of impulse voltages for calibration of digital record but also can used to calibrating the impulse voltage divider.