Abstract: Southwest China karst area is one of the main karst zones in China and also the tour focus in the world. Because of the unique karst landscape and the virgin environment the ecotourism has developed rapidly there in the past 20 years. But lots of environmental challenges aroused and the development decelerated heavily for its special karst environment and environmental vulnerability. Therefore, the karst ecotourism theoretical system is advanced based on former theories and an evaluation model named PRIM model is founded. The PRIM model is made up of 4 parameters （Pollution, Risk, Interaction and Monopolization）, 12 factors and 5 modified indexes. By discussing six tourism elements with PRIM model, the development model to karst ecotourism is brought forward for social/ environmental/ economic efficiency and fit for Southwest China karst region. The ecotourism situation of evaluated object can be indicated by value of Et which is the result of the PRIM model. By the way, primal problems and perfect measures can be brought out by the value of 4 parameters （Pollution, Risk, Interaction and Monopolization）. The PRIM model was applied to evaluate the ecotourism situation of Lunan Stone Forest, Lijiang river and Tian＇edong cave. By the results, Lunan Stone Forest and Lijiang River are normative ecotourism and the value of monopolization parameters is high, but Tian＇edong cave proximate ecotourism and the risk and monopolization parameters are sensitive. The PRIM model is the first mathematical model to evaluate ecotourism in vulnerable karst area, and it is an attempt to set up a mathematical model to develop ecotourism theories. The study is beneficial to karst ecotourism development and the frontier of tourism knowledge.