Objective To explore factors that affect the assessment of sexual self-defense capacity and to evaluate the effect of social functions on sexual self-defense capacity in patients with mental retardation. Methods A 1∶1 matched case-control study was carried out, which included 174 sexual assault cases, 87 with mental retardation and 87 without mental retardation. A record of forensic psychiatry assessment designed by ourselves was used to collect the general characteristics. Wechsler Adult Intelligence-Rerisedin China (WAIS-RC) was used to determine the intelligence quotient. Rating Scale of Intellectual Disability (RSID) and Global Assessment Function (GAF) were used to assess social functions. Results Besides factors such as culture, occupation, knowledge about sex and payment claim, the scores of GAF and RSID were also related to the assessment of sexual self-defense capacity. Moreover, the correct ratio of discrimination was 73.1% (yes), 66.9% (impaired) and 87.2% (no), respectively, and the general correct ratio of discrimination was 78.1%. Conclusion Not only social and legal factors but also the level of social functions should be considered in the assessment of sexual self-defense capacity. Therefore, it might be concluded that multiple discriminant analysis can be useful when assessing the sexual self-defense capacity of patients with mental retardation.