The present invention provides a high-voltage metal-oxide-semiconductor transistor device and a manufacturing method thereof. First, a semiconductor substrate is provided and a dielectric layer and a conductive layer sequentially stacked on the semiconductor substrate. Then, the conductive layer is patterned to form a gate and a dummy gate disposed at a first side of the gate and followed by forming a first spacer between the gate and the dummy gate and a second spacer at a second side of the gate opposite to the first side, wherein the first spacer includes an indentation. Subsequently, the dummy gate is removed.