Electric-field measurement is critical to the investigation of the air gap discharges. The traditional electric-field measurement is intrusive

which may distort the electric field. Therefore

an electric-field measurement system based on electric-field-induced second harmonic generation（EFISH） is set up. The electric field measurement system based on this principle has the advantages of being non-intrusive

not restricted by gas environment

high spatial and time resolution.Moreover

a self-consistent model of the EFISH process is established. The correctness and effectiveness of the proposed model are verified. The spatial distribution of laser field and electric field to be measured are taken into consideration

and the effects of the polarization direction of the laser and the focusing parameters on the second-harmonic signal are studied by simulations and experiments. The working conditions of the plane-wave approximation model is analyzed. The system’s sensitivity is improved by optimizing the optical path

adjusting the Rayleigh range of the laser

and increasing the laser power. It can be realized that the minimum measurable electric-field is 0.4 kV/cm

and the minimum spatial resolution is in mm-grade.