A manufacturing method of a high-voltage metal-oxide-semiconductor (HV MOS) transistor device is provided. The manufacturing method includes the following steps. A semiconductor substrate is provided. A patterned conductive structure is formed on the semiconductor substrate. The patterned conductive structure includes a gate structure and a first sub-gate structure. The semiconductor substrate has a first region and a second region respectively disposed on two opposite sides of the gate structure. The first sub-gate structure is disposed on the first region of the semiconductor substrate. The first sub-gate structure is separated from the gate structure. A drain region is formed in the first region of the semiconductor substrate. A first contact structure is formed on the drain region and the first sub-gate structure. The drain region is electrically connected to the first sub-gate structure via the first contact structure.