Modular steel structures have been developed in the background of industrialization and green development of construction. Widespread attention has been paid to the demountable modular steel structure due to its ability to reduce transportation costs greatly. Joints are key components in demountable structures and have a significant impact on the overall performance of structures. However， demountable joints are often complex in construction and the transmission paths of force are unclear. This paper analyzed the stress performance of the irregular beam-column joint in demountable modular steel structures by completing in-plane bending tests of two joints. It was found that high-strength bolts and plates fastened by them were the controlling factors of joint stiffness. Subsequently， a reliable numerical model was established to further study the rotational performance of the joint in different planes and directions of rotation， and it was determined as a semi-rigid joint according to EC3. Based on numerical analysis and theoretical derivation， approximate calculation models for the initial rotational stiffness and ultimate bending moment of the joint were proposed， respectively. Finally， a power function model was used to describe the semi-rigid performance of the joint， and the approximate model’s moment-rotation curve fitted well with the numerical results.