Abstract: Wellbore rock-mechanics properties become more complicated after the formation water is met in gas drilling, which threatens safe gas drilling greatly. From the perspective of the mechanism of shale hydration, based on the experiment and theoretical calculation, the effect of shale hydration on wellbore rock-mechanics parameters is analyzed. The method for analyzing and evaluating the shale wellbore stability under the condition of formation water production is proposed. Combined with the actual engineering geological characteristics, quantitative evaluation is made on the wellbore stability in gas drilling under the condition that the special and complex formation in western Sichuan produces water. Results obtained from the experiment and the theoretical calculation show that well bore instability and failure easily occurs in gas drilling, because shale is widely distributed in the intervals ranging from the medium shallow one to the medium deep one, and the formation water is abundant in western Sichuan, the problem of wellbore stability is outstanding in the process of gas drilling. The formation water production of the well interval to be drilled and the vertical distribution law of shale formations should be fully recognized during the process gas drilling demonstration, design, and construction.