A trench power semiconductor device and a manufacturing method thereof are provided. The trench power semiconductor device includes a substrate, an epitaxial layer disposed on the substrate, and a gate structure. The epitaxial layer has at least one trench formed therein, and the gate structure is disposed in the trench. A gate structure includes a lower doped region and an upper doped region disposed above the lower doped region to form a PN junction. The concentration of the impurity decreases along a direction from a peripheral portion of the upper doped region toward a central portion of the upper doped region.