The traditional information retrieval model only considers the matches of key words in the corpus, so it cannot recall more comprehensive and accurate results in the forestry domain. Aiming to solve the disadvantage above, the model of semantic query expansion based on ontology is presented for forestry domain. The semantic query expansion is modeled using the semantic reasoning and structure of ontology, and improves the recall and precise ratio in information retrieval finally. This model is a supplementary semantic function for the traditional information retrieval model. The experiment proves that this semantic query expansion model can increase the recall and precise ratio in information retrieval of forestry domain.