Abstract: The article concludes that the distribution rule of corrosion pit on Sichuan gas pipelines conforms to Gumbel first kind of probability distribution rule. Authors calculate the maximum depth of corrosion pit possibly existed by means of computer when mapping Gumbel probability on drawings in calculating maximum depth of corrosion pit. The shortcomings of mapping the probability drawing are fundamentally solved by "the maximum depth of corrosion pit prognostication software (MDCpp-Software)", and the value obtained is unique and confidence level is high. Taking the inspection data from bell hole in some of Sichuan gas transmission pipelines for example, a predication is made with MDCpp-Software on possibly existed maximum depth of corrosion pit, and a comparison of predication results is carried out to LMF data including verified results from pipeline repairing. Comparison results show that the field inspection method combined with the statistics method is rational and scientific, which can provide a reliable basis for assessing the residual strength of corroded pipes.