Abstract: Civil capacity is based on the capacity of will.Grounding on the principle, persons in custody ought to be identified as persons of incapacity.This argument's foundation is that, persons in custody cannot timely and completely acquire information which is necessary for them to declare real will as they can do in the free state;meanwhile a certain period of incarceration may produce disastrous influence on their mentality, all of which would create serious damage on their capacity of understanding and cognition, and make it difficult for them to judge or determine empirically and rationally as the same as ordinary persons can do, and that means civil capacity of persons in custody is imperfect.Investigation and study of the psychology, the judicial system and the judicial practice have given forceful support to the argument.