The invention relates to a semiconductor device, and particularly relates to a power device for optimizing the creepage distance and a preparation method thereof. The power device has a chip mounting unit which includes a base and a plurality of pins, and the side-by-side pins are arranged at non-equal intervals and disposed near a lateral edge of the base. A chip is adhered to the base. A plastic package body coats the base and the chip. The plastic package body includes a plastic package extension part which coats at least part of the pins to obtain a good electrical safety distance between the pins so as to improve the voltage creepage distance of the device.