Abstract: Mudstone hydration leads to a reducing rock strength during horizontal well drilling. The drilling fluid density determined depending upon the critical collapse pressure can maintain the wellbore equilibrium only at the initial stage, but fails to sustain the wellbore stability of horizontal wellbore through the drilling process. Based on the linear elastic rock mechanics and the Mohr-Coulomb rule, a model was developed to predict the wellbore collapse angle and depth after mudstone hydration and subsequently, analyze the wellbore damage degree and enlargement rate under different in-situ stress conditions. The results show that the wellbore collapse depth increases with the reducing rock strength, leading to an expanding overall collapse area. With the soaking time, the rock strength and elastic modulus decline gradually, while the Poisson's ratio and internal friction angle rise. Properly adjusting the drilling fluid density in a timely fashion can reduce wellbore collapse caused by reducing rock strength. For an example horizontal well, the correlation between wellbore enlargement rate and rational drilling fluid density was charted using the presented wellbore collapse angle and depth model. On this basis, the drilling fluid density was adjusted properly and quantitatively to effectively migitate the risk of wellbore collapse. This method provides a theoretical basis for wellbore stability evaluation of horizontal wellbore in rocks with reducing strength and design optimization of drilling fluid density.