Assessment technology of the subjective tests is a key technique in automatic examination system; moreover, the hot spot in the implementation of it is semantic similarity computation. On the basis of word similarity computing using Hownet, this paper proposes how to compute sentence similarity with attribute partition and difference method, and apply this algorithm in paragraph similarity computation. Finally, the similarity computation model of automated assessment system is established. Compared to the conventional model of sentence similarity computation, experiments indicate that this improved method is consistent with the process of manual assessment by reducing the computational efforts.