Based on the equivalent model of dielectric structure and the voltage distribution principle of AC and DC voltage in composite dielectric structure，the DC/AC ratio of oil-immersed all-film capacitor，film-paper composite capacitor and dry metallized film capacitor is deduced. The typical value of the DC/AC ratio of the three dielectric structures and their relationship with compaction coefficient of capacitor are obtained by substituting the typical parameters. The rationality of using DC/AC ratio of 2 for all-film capacitor and DC/AC ratio of

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for dry metallized film capacitor is verified theoretically，and the suggestion of adjusting AC/DC withstanding voltage between terminals of all-film capacitors is proposed.