Adjustability is an important function of the magnetic release for modern molded case circuit breakers. Based on virtual prototype technology, an automatic prediction method is proposed to design reasonable reactive spring parameters for this kind of magnetic release. 3-D finite element method is adopted to calculate the static characteristics of the magnetic release. Then the dynamic characteristics of the magnetic release can be simulated taking into account the variation of the spring parameters with multi-dynamics method. The calculation results have been verified by the relevant experiments. It demonstrates that the proposed method is feasible to perform the design task.