The invention relates to an alarm device used for a cable net operation platform. Steel wire ropes at the peripheral ends of the cable net operation platform pass through and are fixed to a corresponding pedestal beam so as to tension and fix the cable net operation platform. The alarm device comprises a moving plate, a sensing component and an alarm, wherein the moving plate fixedly sleeves one side of the steel wire ropes located on the pedestal beam and the side is close to the cable net operation platform, and the moving plate is attached to a side wall corresponding to the pedestal beam;the sensing component is installed on one side of the pedestal beam which is close to the cable net operation platform, a sensing head corresponding to the moving plate is formed on the sensing component, and the sensing head and the side wall corresponding to the pedestal beam have a set distance; and the alarm is electrically connected to the sensing component, and the sensing head of the sensing component is contacted with the moving plate so as to start the alarm to realize an alarm function. A problem that the steel wire rope slip of the cable net operation platform is not easy to find iseffectively solved. A condition that the steel wire rope slides can be timely found, a potential safety hazard is eliminated, manpower is saved and work efficiency is increased.