Abstract: Unconsolidated sandstone reservoirs are characterized by low strength, high porosity, and high permeability, making them prone to wellbore instability and formation damage during drilling. In order to meet the requirements of wellbore stability and reservoir protection in unconsolidated sandstone drilling, a wellbore collapse pressure prediction model was derived based on the fundamental theories of elastic-plastic mechanics, allowing for moderate wellbore collapse. Compared with the traditional model that maintains full wellbore stability, this new model yields lower collapse pressure predictions. Both the traditional model and the proposed model were used to calculate the collapse pressure of W16-14h well in Wenchang Oilfield. Combined with pore pressure and lost circulation pressure analyses, the drilling safety density window for this well was established. Based on the collapse pressure predicted by the traditional model, the drilling fluid density in the horizontal section must be maintained at 1.26 g/cm³ or higher. However, when the allowable collapse angle around the wellbore does not exceed 90°, the drilling fluid density can be reduced to 1.18 g/cm³. This low-density drilling fluid design can reduce the drilling pressure differential in the reservoir section from 6.9 MPa to 4.3 MPa, a decrease of 37.7%. This approach not only ensures overall wellbore stability, but also effectively protects the reservoir.