A common-rail fuel injector for a diesel engine is provided. A valve seat of a control valve of the fuel injector adopts a T-shaped column structure with a long hole (2001c). Hydraulic pressure at a tail portion of a needle valve is controlled by a liquid, employing the long hole of the valve seat of the control valve. A T-shaped pilot valve seat uses a float mounting structure, in which a large end of the T-shaped control valve seat faces downwards, and a long column of the T-shaped pilot valve seat is slidably assembled in a central hole of an injector body. The large end of the T-shaped control valve seat and the central hole of the injector body are provided with seal seat surfaces fitting with each other. Hydraulic pressure at a lower end of the T-shaped control valve seat presses the seal surface of the T-shaped control valve seat against the seal surface on the injector body. A spring of the needle valve does not need to be mounted at a tail portion of the needle valve. When an engine is stopped or first started, a spring of the control valve also acts as a pre-compressed spring of the needle valve, thereby closing the needle valve. When the control valve is closed, pressure at the tail portion of the needle valve is higher than pressure at a front end of the needle valve, and the needle valve is closed by means of a pressure difference.