There exists the possibility of guess and misplay when testing, however, the existing diagnosis models do not take these two factors into account, so it is difficult to diagnose the knowledge states of students effectively. Based on the guess probability and the misplay probability in the knowledge space theory, a new model is proposed which could effectively diagnose the knowledge states of students in that case. To diagnose the real knowledge states of the students, the model transfers the answer states according to the probabilities of guess and misplay of each question, and focuses on the analysis of the calculation of the two probabilities. The model has a good effect on practicability and the veracity of diagnosis.