Tight sandstone reservoirs are distinguished by high compressive strength, low porosity and permeability, and low effective stress coefficient, which contribute to enhanced wellbore stability during drilling. Nevertheless, the rate of penetration is relatively low, and there is a higher likelihood of reservoir damage. The application of underbalanced drilling in tight sandstone reservoirs serves as an effective means of enhancing drilling rate while simultaneously protecting the reservoir. However, if the underbalanced pressure differential is too large, wellbore instability can occur, which may pose a risk to drilling safety. Engineering practice and experimental data have demonstrated that the theoretical calculation results of traditional wellbore collapse pressure underestimate the ability of borehole to remain stable in tight sandstone reservoirs. To ensure the overall stability of the wellbore during the underbalanced drilling process, a new collapse pressure calculation model has been developed through rock mechanics theoretical analysis. This model can serve as the mechanical basis for the design of underbalanced drilling pressure differentials. A horizontal well in a deep tight gas field reservoir, located offshore, was subjected to an analysis. In the normally pressured reservoir section at a true vertical depth of 3633 meters, the minimum drilling fluid density required to maintain wellbore stability is 1.11 g/cm3, which is not suitable for underbalanced drilling. In the reservoir section with an high pressure at a true vertical depth of 4355 meters and the pressure coefficient of 1.28, the minimum allowable drilling fluid density is 1.16 g/cm3, which meets the mechanical conditions for implementing underbalanced drilling.