When the HVDC transmission lines cross over agricultural greenhouses, the space charges accumulate on the dielectric film. An experiment platform is set to research the influence of conductor voltage and space charge distribution on the charge-dissipation characteristics of dielectric films. Experiments show that charges accumulate on the film surface, leading to ground-level electric field distortion. In Laplace field, there is a charge accumulation on the film surface with a polarity opposite that of the conductor. Space charges accumulate on the film surface in the ion flow field, enhancing the ground-level electric field below. The saturation charging time of the film is negatively correlated with the electric field. The negative polarity charge of the PE material film dissipates faster than the positive polarity. In addition, the electric field is the direct factor affecting film charging, and the space charge distribution is the indirect factor. The charging mechanism for gas-side conduction involves two parts: the blocking effect of the dielectric film on the ion flow and the accumulation of space charge on the film surface until the external Laplace field is zero. This study provides experimental data and theoretical support for establishing a calculation method for the ion flow field with dielectric film charging.