According to array radar system, low-angle tracking problem was investigated from the perspective of revised mono-pulse. Firstly, based on the array signal model under multi-path conditions, the double-null mono-pulse algorithm was deduced by using the principle of Maximal Likelihood (ML) estimation. The revised sum-difference patterns were designed by digital beamforming (DBF). Simulation result shows the relation of double-null mono-pulse performance, SNR, target elevation, and complex reflection coefficient. Finally, measured data is used for validation. Result shows that the root of mean square error (RMSE) is still less than 1/40 of the beamwidth when the target elevation is less than 1/4 of the beamwidth. Therefore, the double-null mono-pulse algorithm is effective for low-angle tracking problem.