Asteroid impacts pose a major threat to all life on Earth. Deflecting an asteroid on an impact trajectory is critical to mitigating this threat. A kinetic impactor remains the most feasible asteroid deflection method. However, due to launch capability constraints, an impactor with a limited mass can only minimally change the velocity of an asteroid. To improve the deflection efficiency of the kinetic impactor strategy, this paper proposes the Assembled Kinetic Impactor (AKI), which combines the spacecraft with the launch vehicle upper stage. After the launch vehicle upper stage sends the spacecraft into an Earth-escaping trajectory, the spacecraft-rocket separation is not performed, and the spacecraft controls the AKI to impact the asteroid. By retaining the mass of the launch vehicle upper stage, the mass of the impactor is increased, thereby enhancing the deflection efficiency. According to the technical data of the Long March 5 (CZ-5) launch vehicle, missions to deflect Bennu are designed to demonstrate the power of the AKI concept. Simulation results of the AKI compared with the Classical Kinetic Impactor (CKI, with spacecraft-rocket separation) show that the addition of the mass of the upper stage increases the deflection distance by >3 times. To achieve a given deflection distance, the addition of the upper stage mass reduces the number of launches to 1/3 that of the number of CKI launches. The AKI concept makes it possible to deflect large Bennu-like asteroids with a nuclear-free technique with a 10-year launch lead time. Additionally, with a single CZ-5, the deflection distance of a 140-m-diameter asteroid with a 10-year launch lead time increases from less than 1 to more than 1 Earth radius, representing an improvement in the reliability and efficiency of asteroid deflection missions.