The characteristics of the induced voltage of the high power disk amplifier, which is not grounding (suspended), is researched experimentally. Results show that the peak value of the induced voltage is dominated by the capacitance between the flash lamp and the cavity and the number of flash lamps in the amplifier, and the independent cavities don′t affect each other directly. Exprimental study on induction properties of the amplifier under a variety of grounding methods is conducted, and a grounding plan of wholly grounding the amplifier with the internal and external cavities electrically integrated is demonstrated, which is successfully applied to the 350 mm disk amplifier of the ninth beam of SGⅡ. The possibility of electrical breakdown inside the amplifier is eliminated radically, and the induced voltage of the amplifier is reduced to 18% of that when the amplifier is suspended. As a result, the reliability and safety of the disk amplifier are improved obviously, the compactness and cleanliness of the amplifier are also improved simultaneously. Since this grounding plan is adopted, the 350 mm amplifiers have operated reliably and safely for more than 400 shots without any electrical breakdown and interference.