Abstract: Formulas of reliability of wellbore integrity are derived through central-point method and comparatively verified by the Monte-Carlo method.Built upon these formulas, relationship between “wellbore integrity reliability probability(WIRP)” and “wellbore integrity reliability index(WIRI)” is established and expatiated.On this basis, the applicability and accuracy of the proposed method is verified by case of T4.The results show that the WIRP of T4 is 0.8966 and the integrity grade is “minor reliable” using the proposed method in this dissertation, while the WIRP of T4 is 0.9106 and the integrity grade is “reliable” using Monte-Carlo method.It is proved that the evaluation method proposed herein is accurate and relatively conservative which is more suitable for practical application.The achievements of this dissertation are expected to provide the guidance for the design and safety evaluation of gas wells.