During an earthquake, the brace of a multistorey X-braced steel frame experiences out-of-plane instability. After the brace becomes unstable, the middle span of the brace-intersected beam exhibits a large imbalance force. This force causes a significant vertical displacement in the middle of the brace-intersected beam. To address this issue, an improved multistorey X-braced steel frame is proposed. Adding an arc energy-dissipation structure to the original structure adjusts the bearing capacity of the brace and ensures that the brace does not lose stability. The deformation of the arc energy-dissipation structure reduces the vertical displacement of the brace-intersected beam. A finite element model of the improved multistorey X-braced steel frame is established, and the seismic performance of the structure is analysed. The analysis shows that the brace of the improved multistorey X-braced steel frame does not experience instability, and the lateral bearing capacity is increased by 20%. The vertical displacement in the middle of the brace-intersected beam is small. The improved multistorey X-braced steel frame exhibits good seismic performance.