The guidance precision of the launch vehicle during the flight stage within the atmosphere is greatly affected by the guidance coefficients of perturbation guidance. However, there is no comparable database for the guidance coefficients selection in the argumentation of a new launch vehicle, which cannot meet the demand for rapid calculation using the traditional test method. A real-time calculation method for the guidance coefficient which considers both the real-time status and the follow-up flight trajectory is proposed, based on introducing the concept of remaining flight speed in the standard trajectory. Through the numerical simulation, it is proved that higher guidance precision is achieved by the proposed method in comparison with the traditional method with constant coefficients. Since the guide capacity was predicted by this method, the guidance instruction do not change dramatically, which contributes to easy design for the attitude control system. Meanwhile, a fully analytical method was developed, so there is no additional calculation requirement for the rocket-born computer, giving rise to convenient engineering realization.