From a viewpoint of the sectirity and the exactness for a fire system,this paperexplicates the function and the element of two-degree alert(TD),pre-alert and up-alert applied tothe detection and the control in the fire system. An example in the paper deduces the logical expres-sion designing TDA and the hardware logic circuit is given. The paper also presents the software pro-gram realizing TDA utilizing the single chip microcomptiter(87C48),