The invention discloses a fuel injector capable of injecting fuel to an engine quantitatively at fixed time in a controllable manner. The fuel injector comprises a fuel injector body, a piston couple, a fuel nozzle return spring component, a fuel nozzle couple, a transition plate, a throttling pore plate, a ball valve, a control valve couple and an electromagnetic valve component, wherein the piston couple and the fuel nozzle return spring component are installed in the fuel injector body; the fuel nozzle couple and the transition plate are installed on the lower end of the fuel injector body; the throttling pore plate, the ball valve, the control valve couple and the electromagnetic valve component are installed on the upper end of the fuel injector body. The hydraulic force in a pressure control chamber is controlled by opening and closing a fuel discharge throttling hole in the throttling pore plate by the ball valve, and further, the hydraulic force of the upper end of a needle valve rod is controlled; a fuel nozzle is opened and closed by the unbalance of the hydraulic force at the upper end and the lower end of the needle valve rod so as to control fuel injection precisely. The fuel injector provided by the invention is high in controlled pressure, can be used for injecting fuel quantitatively for multiple times, has the characteristics of high-speed response and large pressure control range, and can control the fuel injection quantity, the fuel injection timeliness and the fuel injection pressure accurately and flexibly and improve the combustion in an engine cylinder, thereby reducing harmful emissions of the engine, and improving the economical efficiency, the dynamic property and the drivability of the engine.