Abstract: To study the effect of low-frequency magnetic field on the solution ice crystal formation process under the condition of slow temperature-dropping rate, a device to produce alternated magnetic field was designed by using switching capacitor. This device, which is composed of oscillator, switching capacitor, second-order tracking system, digital switching system, power-amplifier circuit and Helmholtz coils, can produce rotating magnetic field whose frequency and strength can be adjusted continuously. The experimental results of 0.9% KMnO₄ solution with a slow temperature-dropping rate show that low-frequency rotating magnetic field markedly affects the ice crystal formation process and the size of ice crystal lessens with the increase of magnetic field frequency. This preliminary study indicates that low frequency crystal magnetic field is significant in organism cryopreservation.