A transient force was observed with high-voltage capacitors switch-on/off by several teams with dozens of experiments. It is well-known that, in the maxwell equations, as an electric field is switched on/off, an instantaneous superstrong magnetic field can be produced. For the hypothesis of gravitational-magnetic field interaction, the superstrong magnetic field should make a transient variation of the gravitational acceleration. Therefore, these experiments could be solid evidence for gravitational-magnetic field interaction. And, a way to finally judge the reason for the transient force is presented.