Abstract: Rapid intensification (RI) remains a difficulty in typhoon intensity prediction currently. In order to enhance the understanding of the mechanism of typhoon intensity variation and improve the typhoon intensity prediction accuracy, the characteristics of the upper-tropospheric environmental field of typhoon Hagupit (2020) during the RI phase were analyzed using the CMA best track data and ERA5 reanalysis data, and the possible mechanisms of the upper-tropospheric environmental field affecting typhoon intensity were discussed. The results show that: (1) The upper-level weather systems affecting the intensity variation of Hagupit (2020) mainly include the South Asian High, the Tropical Upper-Tropospheric Trough, and the mid-latitude trough. (2) The upper-tropospheric environmental field affects the typhoon intensity by the interaction with the upper-level outflow of the typhoon. On one hand, the favorable uppertropospheric circulation enhances the upper-level outflow by propagating the eddy flux convergence of relative angular momentum towards the typhoon center, thus promoting the secondary circulation formed by the outflow and ascending motion. On the other hand, the enhanced outflow serves to divert the environmental flow, thus reducing the vertical wind shear. Therefore, the ventilation effect weakens and the typhoon intensifies. (3) On the 360-K isentropic surface, the superposition of small-scale positive potential vorticity (PV) anomalies on the typhoon circulation can enhance the typhoon intensity. At the same time, the "high-PV pool" in the stratosphere connects with the positive PV anomaly region in the middle troposphere, which can promote the formation of the upper-level warm core and intensify the typhoon. The comprehensive analysis of the upper-tropospheric environmental field can provide some reference for typhoon intensity prediction.