The diseases of existing tunnel lining structure threaten the traffic safety, traffic quality and tunnel service life. Starts from 13 independent influencing factors of existing tunnel lining structure diseases from 3 aspects of engineering construction, natural environment and engineering geology, the index system of forming factors of diseases of existing tunnel lining is established by AHP method, and the diseases evaluation method for existing tunnel lining structure is proposed based on AHPExtenics mode firstly. And then, the weight of safety evaluation index is determined by combining engineering practice and expert scoring, and the safety evaluation index and weight are extended by using correlation function. Finally, the disease evaluation grade of existing tunnel lining structure is obtained. In this method, the traditional fuzzy comprehensive evaluation method is replaced by the formal extension comprehensive evaluation model, and the qualitative analysis results are given in quantitative form, which makes the calculation results more objective and provides a new method for the diseases evaluation of existing tunnel lining structure.