The valve-side outing line device of converter transformer (CT) is composed of oil-impregnated paper and insulating oil channels. With the “green and low-carbon” transformation and development of the power industry, environmentally friendly natural esters witness more frequent use in CT. The space/interface charge problem is the key factor affecting the performance of the valve-side outing line insulation. In this work, a simulation model of a one-dimensional natural ester oil-paper insulation is established, and the characteristics of space/interface charge distribution evolution under different electric field strengths and temperatures are calculated. The simulation results show that within the range of simulation parameters, the steady-state space/interface charge density will increase with an increase in either electric field strength or temperature, and the variation of electric field strength will affect the time when the space/interface charge density reaches the steady state.