Focusing the impact speed constrained guidance problem under drag uncertainty, this paper proposes an impact speed constrained guidance law based on optimal error dynamics. Firstly, this paper analyzes the effect of the normal maneuver of vehicle on impact speed; secondly, a disturbance observer is designed to address the drag uncertainty, which can observe the drag during the initial flight phase and enhance the robustness of the guidance law; finally, an impact speed constrained guidance law is designed based on optimal error dynamics, and a comparative simulation experiment with a speed correction based guidance method is conducted. The simulation results show that the guidance method proposed in this paper can achieve precise guidance and accurately control the impact speed under drag uncertainty, demonstrating great reliability and potential for engineering application.