Miniature Circuit Breaker (MCB) is widely used in terminal power distributions and civil buildings, its annual production has more than 500 million poles. However, in terms of the short circuit protection of MCB, and there's no short circuit delay function, so it can not reach the full selective protection, even through the cooperation between the time selection and the current selection. In paper, a new structure of MCB’s electromagnetic tripping device is proposed, which is able to realize short-circuit delay protection. The new is on the traditional structure added a secondary winding, then controlling the on or off of the secondary winding, using its demagnetization effect to realize the short time delay function when the secondary winding is closed. This is a new idea in the field of low-voltage circuit breakers. In addition, take the U type electromagnet as an example, through the analysis of the magnetic circuit, the main factors that affect the magnetic effect of the secondary winding are studied by using MATLAB.