A lithium niobate optical waveguide-based integrated electro-optic (EO) electric field (E-field) sensor dedicated to the measurement of intense nanosecond transient electromagnetic pulse (EMP) signals has been developed and calibrated. The time domain calibration system for measurement of intense nanosecond EMP signals has been established. A pure optical bias phase angle control system based on wavelength tuning has been developed and implemented to ensure that the sensor has a linear transfer function. The fluctuations of the sensor static output optical power are <0.1 dB with the proposed bias control system while <3 dB without bias control. The time domain characteristics of the detected pulsed E-fields have been compared with those of the input EMP signals. For the first type nanosecond level (ns-level) EMP signal, the relative errors of the detected E-fields on rise time, fall time, and pulse width are 0.38%, 0.69%, and 0.79%, respectively. Also, for the second type ns-level EMP signal, the relative errors of the measured E-fields on rise time, fall time, and pulse width are 0.40%, 0.31%, and 0.01%, respectively. All these results demonstrate that the developed integrated EO E-field sensing system has the potential to be used to accurately extract the information of transient E-fields.