Cause analysis and depth calculation of rock cell can significantly help demonstrate the weathering mechanism of the soft rock of the rock cell. However, the current methods of analyzing weathering mechanism attach little importance to the mechanical environment of rock. Damage mechanics typically involves the calculation of a damage factor. To explore the weathering mechanism of soft rock at the lower part of gently inclining rock slope, this study based on the calculation method of a damage factor in damage mechanics, proposed a calculation formula for the damage factor of soft rock under the combined effect of pressure and temperature and the method for fatigue rupture. Then, taking the mudstone of a single rock cell in Hongyan Mountain of Jiangjin in China as an example, the calculation formula and method for soft rock were used to analyze its weathering mechanism. The analytical parameters included weathering rate, damage, and fatigue rupture of soft rock. Results show that the maximum influencing depth of temperature on soft rock is approximately 1.00-1.20 m, and the weathering rate of the soft rock is faster at the beginning of temperature fatigue loading. Weathering rate slows down with the fatigue number increasing. At the lower-level temperature difference, the weathering process of soft rock accelerates when the rock shifts from low to high temperature. However, when the soft rock is in large temperature difference, the weathering process accelerates at the relatively lower rate. The damage of the soft rock is linearly related to the pressure from the upper hard rock. Conclusions of this study provide a theoretical basis for the prevention and treatment of collapsed hazards of gently inclining rock slope.