Abstract: Aimed at the high ontology complexity and the low ontology reuse efficiency of the independent ontology in the security field, a modular ontology reuse algorithm is proposed. In order to determine the sub-module concept of the upper and lower relations, the distributed description logic (DDL) language was applied to build a modular ontology in the security field. On this basis, the modular ontology reusing algorithm based on the conservative expansion theory was used to realize the module extraction and reuse of the modular ontology. The algorithm was evaluated in the built ontology library, which increased the proportion of the reused ontology in the reused target ontology and reduced the extraction time of the ontology reuse module. Experimental results show that the algorithm can reduce the complexity of the reused target ontology and effectively realize the reuse of modular ontology.