In the process of handling the case by the prosecutor, the nature of the theft case is not accurate, and the lack of experience in the sentencing suggestion leads to insufficient accuracy of the sentencing recommendations. In order to enable the prosecutor to give more accurate sentencing recommendations, provide an auxiliary sentencing reference, through the collation and analysis of the legal documents theory and knowledge system of the theft case, excavate and reason the implicit relationship and deep relationship, and build the ontology model. This study proposes a method for constructing knowledge graphs of legal documents based on ontology, and designs custom inference rules. It realizes the knowledge of the legal documents of theft cases in similar sentencing cases, and obtains the ideal test results. The research proves that the knowledge graph of the legal documents based on ontology-based theft cases is constructed, and the intelligent reasoning technology is used to provide the prosecutor with a similar case sentencing reference, which assists the prosecutor to give more reasonable sentencing suggestions.