Abstract: In order to solve the problems of fog horn control equipment aging, spare parts outage and high failure rate of "KanTan IV"semisubmersible offshore drilling platform, the composition of the fog horn control system, the principle of the control equipment and the method of improving reliability are discussed. Based on Siemens LOGO controller, the original control system is reformed, which effectively solves the original fault of the fog horn and saves the cost of replacing the integrity fog horn. This paper can provide reference for the fault resolution of the similar fog horn and the transformation and design of the control system.