Abstract: Though several methods has been used to suppress the background noise and eliminate the scene influence for the effective ranging capability and angular accuracy demarcating detection,these methods still can not meet the practical requirement for the target at sea.The calculating method of the infrared demarcating theory is defined.Through validation,this method is proved to be reasonable and convenient for application.In order to resolve the diffculty of infrared demarcation and remedy the equipment deficiency, this method is proposed.