The voter in the mimic Web server calculates the similarity of the heterogeneous executor response webpage in order to judge whether the response is legal output and thus to prevent webpages tempering. At present, the voter treats entire Webpage as a string and uses the string edit distance to calculate the similarity of the webpages. In this way, it caused problems such as large amount of calculation, ignorance of the original structure information of the webpages, and so on. In this study, the improved simple tree matching method is used to calculate the similarity of the webpages by calculating the similarity of the DOM tree of the webpages. The matching degree of DOM tree node is determined by the editing distance of the node string. The proposed algorithm is applied to the mimic Web server to verify the webpage tamper. Compared with the existing algorithms, the algorithm used in this study not only adapts itself to the heterogeneous but also improves the efficiency and accuracy of the voter.