Abstract: Spatial electric field diagnostics of atmospheric-pressure pulsed streamer discharges are crucial for accurately modulating streamer discharge characteristics and unveiling plasma instability mechanisms. The electric field induced secondary harmonic method has advantages of non-intrusiveness and high spatial-temporal resolution. However, effects of time-varying non-uniform electric field on the secondary harmonic generation are unclear. Meanwhile, the actual electric field calibration factor may be dependent on the streamer discharge stages. Secondary harmonic signal generation and effects of the external electric field profile are analyzed. Sensitivities of the secondary harmonic signal on the time-varying non-uniform electric fields of different streamer discharge stages are analyzed based on the plasma fluid modelling. Furthermore, based on the nanosecond pulsed laser secondary harmonic generation measurement platform, spatial axial electric field evolutions of single repetitively pulsed negative streamer are experimentally obtained in a combined electrode structure. Effects of the pulse repetition frequency on the spatial electric field are unveiled. Several suggestions of improving E-FISH measurement are proposed based on time-varying non-uniform electric fields of streamer discharges.