Abstract: The presence of water vapor in air will change the dielectric properties of the gas, which will seriously impair the external insulation properties of transmission lines. In this paper, the effect of humidity on streamer discharge is studied theoretically and experimentally. Theoretically, the discharge parameters of wet air are corrected by the Wieland approximation method, the discharge parameters of dry clean air and wet air are compared, the discharge parameters of needle plate air gap streamer discharge under different humidity are calculated, and the streamer initiation, propagation criteria and breakdown voltage calculation method under wet air are proposed. In terms of experiments, a streamer discharge chamber is built, and the bar-plate gap streamer discharge experiment is carried out. The experimental results are in good agreement with the theoretical calculation results. The wet air discharge parameters show that, under a low electric field (E/p < 0.3 V/(cm·Pa)), the increase of humidity has a negligible effect on the ionization coefficient; under the condition of a strong electric field, humidity has a significant promoting effect on the ionization coefficient. The adhesion coefficient increases with the increase of humidity under a low electric field, and decreases with the increase of humidity under a strong electric field. The calculation results of streamer in pin-plate gap show that the increase of humidity will increase the peak electron density and radius of streamer, and accelerate the propagation speed of streamer. The bar-plate breakdown experiment and the calculation results of breakdown voltage show that, when the humidity is low, the humidity has a negligible effect on the breakdown voltage. When the relative humidity reaches about 94%, the increase of humidity will significantly increase the breakdown voltage.