The present invention discloses a high voltage device and a manufacturing method thereof. The high voltage device is formed in a substrate. The high voltage device includes: a gate, a source and drain, a drift region, and a mitigation region. The gate is formed on an upper surface of the substrate. The source and drain are located at both sides of the gate below the upper surface respectively, and the source and drain are separated by the gate. The drift region is located at least between the gate and the drain. The mitigation region is formed below the drift region, and the drift region has an edge closer to the source. A vertical distance between this edge of the drift region and the mitigation region is less than or equal to five times of a depth of the drift region.