For the strapdown airborne gravimeter measurement system, the extended Kalman filtering equation of the GPS/Strapdown inertial integrated navigation, the gravity anomaly model of secondorder Markov process and the Kalman filtering state equation of state which takes the gravity anomaly as variable are given. The measured data of Strapdown airborne gravimeter are processed by using the smoothed forward and backward Kalman filtering method, and the freeair gravity anomaly values are obtained. The results are compared with those of the GT2A airborne gravimeter equipped on the same plane. The comparison results show that the difference between the freeair gravity anomaly values is less than 0.85 mGal.