After making landfall, Typhoon In-Fa (No.2106) experienced a slow decline in intensity, persisted on land for more than four days, and caused heavy damage. This study utilizes a series of numerical experiments with sensitive parameters to investigate the potential causes for the prolonged duration of the typhoon. Results indicate that the large size of Typhoon In-Fa and the substantial transfer of surrounding water vapor played significant roles in its persistence after landfall, with the latter being especially crucial. Specifically, after landfall, the core region of the typhoon continued to receive abundant moisture. Strong convective activity within the eyewall initiated widespread latent heat release through condensation, which in turn facilitated the development and maintenance of secondary circulation (low-level inflow and high-level outflow). This process enhanced the vertical coupling between the higher and lower levels, thereby slowing down the weakening of the typhoon. Furthermore, the abundant water vapor in the boundary layer inflow reduced both the lifting condensation level and the free convection height within the core region. The active convection activity also delayed the upward development of the low-level cold core structure, which helped sustain the warm core structure of the typhoon.