Oil-impregnated power capacitors are currently the most widely used power capacitors, characterized by low cost, low dielectric loss, and high reliability. With the development of transmission systems in high-altitude areas, the reliability of oil-impregnated capacitors in plateau environments has been greatly challenged, mainly brought by the failure caused by the deterioration of partial discharge performance. Therefore, this paper proposes to study the partial discharge performance of capacitor impregnation fluid under different temperatures and pressures. The results show that both the increase in temperature and pressure lead to an increase in partial discharge inception voltage (PDIV), as well as a decrease in discharge magnitude, discharge frequency, and repetition rate. This is mainly due to the changes in gas solubility and viscosity. This paper provides supports for improving the performance of oil-impregnated capacitors in high-altitude areas.