Abstract: To improve understanding of essential aspects that influence forecasting of tropical cyclones, through an ensemble simulation of typhoon Hagupit (2020), this study investigates the impact of the initial vortex and environmental factors on the peak intensity of Hagupit. It is found that there is a stronger correlation between the peak intensity and the maximum azimuthal mean 10 m tangential wind speed than the peak intensity and the maximum 10 m wind speed. This is ascribed to the prominent asymmetry of Hagupit at the early developing stage. Due to the asymmetry, the maximum 10 m wind speed cannot represent the entire wind speed of the vortex. In contrast, the maximum azimuthal mean 10 m tangential wind speed reflects the storm entire wind speed properly and thus is a better predictor for the peak intensity. A stronger correlation exists between the outer-core size and the peak intensity than the inner-core size and the peak intensity. Comparing to the size of the outer-core and the inner-core, the fullness of the typhoon has the strongest and significant correlation with peak intensity, the larger the fullness, the stronger peak intensity. The environmental flow is also found to affect the peak intensity of Hagupit substantially. In the ensemble members with stronger typhoon Sinlaku (2020) and subtropical high respectively to the west and east of Hagupit, the storm usually moves northwards faster under the effect of the steering flow with larger northward component. As a result, the intensification of Hagupit is inhibited by the earlier intrusion of dry air from the north and larger vertical wind shear. These findings mean that for typhoons such as Hagupit, a precise description of the initial vortex structure, nearby typhoon, and subtropical high are crucial for improving the prediction of the maximum intensity.