The basic task of an airborne weather radar is to detect the position and intensity of meteorological targets ahead of the plane. The intensity of the meteorological targets can be represented by the physical quantity called reflectivity factor, which is often estimated from the weather radar equation. When observing the middle or long r a nge meteorological targets at low altitude from a high altitude radar platform, due to the incomplete beam filling, the measurement result of the reflectivity f actor is much less than the actual value. This effect seriously lowers down the accuracy of determining the hazard level of the target. This paper fi rstly introduces a beam filling coefficient into the weather radar equation and t hen presents its estimation method based on the real time radar operating parame t ers. The variation of the filling coefficient is fully analyzed under different radar system parameters. Finally, the validity of the proposed method is verifie d by processing the raw data of the simulated meteorological targets. Simulatio n results show an effective improvement of the accuracy of the reflectivity facto r.