Local thinning of submarine pipeline always exists due to the complexity and dynamic of marine environment loads. However, the location and extent of thinning areas are random. Mechanical research on the randomness of local thinning of submarine pipelines is significant for pipeline’s stress analysis and failure evaluation. In this paper, finite element model of locally thinned submarine pipeline is established and based on the Monte-Carlo method, the effect of input random variables on the submarine pipeline’s reliability is analyzed. Probability distribution ranges of equivalent stresses are got and further, relationship equation between confidence level and stress limit has been obtained. The result more accurately reveals the relationships of locally thinned pipeline and its elastic modulus with equivalent stress, which can provide foundation for safety analysis of locally thinned submarine pipeline with internal pressure.