There is dispute on whether antecedent actions are the source of act duty in omission crimes.Considering the difference between the cause power of act and omission and the need to punish crimes,we should admit that antecedent actions are the source of act duty except under some special conditions.Antecedent actions should be legislated,and its substantial basis is that the actor′ dangerous antecedent actions make him have the duty to protect the victim because the relationship between the actor and the victim requires the actor to prevent a danger to happen.