Abstract: Planar array radar is one of the most important radar forms in radar systems.An adaptive digital beamforming algorithm for planar phased array system is studied in this paper.The element number and the data quantity of the planar array are quite large,so it is difficult to apply DBF to planar array,and the corresponding study is few.SMI algorithm based on planar array is recommended,for the algorithm can operate in high speed.In order to improve the performance of the sidelobe of the algorithm,diagonal loading(DL) technique could be applied.The selection of the DL value is very important. This issue is discussed in detail in the paper,and it is proposed that the diagonal loading technique can limit the influence of the channel error of the array on the performance of the algorithm.