In this paper, several crop seeds were selected as representatives to be treated by the high voltage electrostatic field. Through determining the content of free radicals, the original mechanism of the biological effects of the electrostatic field on crop seeds was discussed. In this experiment, barley and beet seeds were selected, which were divided into 2 groups at random with similar numbers in each group. One is the control group, the other is the experimental group. The experimental group was put into an even intensity electric field produced by a parallel polar plate. The barley seeds were treated by 4 kV/cm for 10 minutes, while the beet seeds were treated by 4.5 kV/cm for 10 minutes. The seeds (both the control group and experimental group) were milled into flour, then the same amount of flour was put into the sample tube, and was put into the ESR(electronic spins resonance) equipment to determine the content of free radicals. The result of the experiment shows that the electrostatic fields with a certain intensity can increase the content of free radicals in seeds. It is proposed that the original mechanism of the biological effects of the electrostatic fields on crop seeds is that the electrostatic fields affect the activity of seeds through increasing the content of free radicals in seeds by affecting the water molecules inside seeds and some large biological molecules.