The invention discloses a second-order side channel energy analysis method for an SM4 algorithm of a simple mask. To achieve mask protection for the SM4 algorithm with the output of four S boxes carrying the same mask value, the method comprises the steps of (1) acquiring energy traces and establishing a sampling energy consumption matrix; (2) preprocessing the energy consumption matrix in the previous step; (3) selecting attack points to conduct CPA or DPA to obtain rki, 0; (4) re-selecting an S-box operation part and following the steps (2) and (3) to obtain rki, u through analysis; and (5) carrying out the steps (2), (3) and (4) on previous/next four rounds to obtain a round key and obtaining an initial key through a key inverse expansion operation. The technical scheme of the invention innovatively provides the new second-order energy analysis method, develops a new situation of SM4 cryptographic algorithm attack, extends the side channel power analysis method for the SM4 algorithm, and can fully and effectively conducts side channel energy analysis on the SM4 cryptographic algorithm; and the method of the technical scheme is highly practical for common mask protection measures.