The traditional diagnostic model has not considered that there exists the possibility of irrational mistakes when testing. While it is an indispensable factor in the actual test to effect the result of one’s test. The existing diagnostic model has combined the knowledge space theory and diagnosis to get the state of knowledge subjects. But with the continuous development of measurement theory, there is growing concern about the skill level of testee. This article applied diagnosis to Extension of Knowledge Space and proposed a diagnosis model with extension of the knowledge space that uses the formula in item response theory. The diagnosis model could still effectively diagnose the skill level of testee when those mistakes happened.