The present invention provides a voltage conversion device and a method for adjusting common mode noise impedance, which relates to the circuit field, and enables a common mode impedance value of a noise source and an impedance value of an EMI filter to enter a mismatch state, so as to reduce a restriction on design of the EMI filter, so that a size of the EMI filter is smaller and utilization efficiency of the EMI filter is higher. The method is: adjusting a common mode impedance value of a noise source by adjusting a balanced impedance value in a balanced winding on a voltage conversion device, so as to enable the common mode impedance value of the noise source and an impedance value of an EMI filter to enter a mismatch state.