Abstract: Partially encased steel-concrete composite columns (PEC columns) have broad application prospects in the field of prefabricated buildings. In recent years, rectangular PEC columns have been widely used in the residential field, but L-shaped PEC columns, especially in frame-support structures, are less used. L-shaped PEC column is a new type of composite structural member. Because its limb thickness is the same as that of the wall, it avoids indoor convex columns, so it has better application prospects in prefabricated steel structure residential buildings. However, the structure of box steel support joints is more complicated. Usually in the project design, the connection structure is determined by referring to the steel structure joint structure in the 16G519 Multi-High-Rise Civil Building Steel Structure Joint Structure Detail Map. Therefore, the research on the connection structure and performance of L-type PEC column and support needs to be improved. Based on the background of Dongshenghefu 10# building residential project, according to the seismic design requirements of “strong joint and weak component”, the internal force of YJK model is extracted according to the requirements of elastic analysis under moderate earthquake: under the condition combination of 1. 3(D+0. 5L) +0. 3W+1. 4Eh, the maximum axial pressure of the support is obtained, and the maximum axial tension of the support is obtained under the condition combination of 1. 0(D+0. 5L) +0. 3W+1. 4Eh. According to the internal force of the relevant members under the combination, the finite element analysis of the traditional scheme and the optimization scheme of the box steel support joint is carried out. Based on the results of the finite element analysis, the stress distribution and stress value of the concrete and the main steel parts under tension and compression are analyzed. It is concluded that the pressure from the abdomen of the box support can be resisted by the concrete filled in the L-shaped PEC column, and the tension can be effectively transmitted through the horizontal stiffening plate. In addition, the stress distribution and stress peak of the optimization scheme are basically the same as those of the traditional scheme under the combination of medium earthquake elastic conditions, and the safety margin of the connection is good, which meets the performance requirements of the “medium earthquake elastic”combination conditions. The performance of the box-type support node structure of the optimized L-shaped PEC column has not decreased significantly, but it brings convenience to the processing and improves the quality controllability.