The coincidence between the atom trajectory and the Raman pulse sequence is very important for an intersection type atom interferometer. Here we present a feedback control technique for the atom trajectories in our horizontal gravity gradiometer, which improves the stabilities of the trajectories by about 2 orders of magnitude. Through the further study of the dependence of the interferometer contrasts on the atom trajectories, we lock the trajectories at optimal positions. And by this technique, the sensitivity of the gravity gradiometer is improved from 982 E/Hz^1/2 to 763 E/Hz^1/2, while the long-term stability is enhanced more significantly and reaches 8.9 E after an integration time of 6000 s. This work may provide hints to other experiments based on intersection type atom interferometers.