A feasible guidance scheme with impact time constraint is proposed for attacking a sta-tionary target by missiles with time-varying velocity. The main idea is to replace the constant veloc-ity with the future mean velocity; therefore, the existing time-to-go estimation algorithm of the proportional navigation guidance law can be improved to adapt to varying conditions. In order to obtain the prediction of the velocity profile, the velocity differential equation to the downrange is derived, which can be numerically integrated between the current downrange and the target posi-tion by the on-board computer. Then, a third-order polynomial is introduced to fit the velocity pro-file in order to calculate the future mean velocity. At the beginning of each guidance loop, the future mean velocity is predicted and the time-to-go information is updated, based on which a novel biased proportional navigation guidance law is established to achieve the impact time constraint. Finally, numerical simulation results verified the effectiveness of the time-to-go estimation algorithm and the proposed law.