In order to study the mechanical behaviors of irregular joints in rigid steel frames which had been used in largescale power plant, 6 specimens in 1/4 scale of the prototype model were designed according to different axial compression ratio and section height ratio of beams and were tested under low cyclic reversed loading.Based on the failure law of specimens, the main factors that affect the bearing capacity of the irregular joints were analyzed.By the division of panel zone, the calculating formula was put forward according to mechanical principles.It is found that the difference between calculating results and experimental results is about 18%.And the proposed method for calculating the irregular joints in rigid steel frames is feasible.