To promote construction industrialization and improve the seismic performance of moment resisting frames with concentrically brace， a new type of partially encased composite （PEC） brace is proposed. Low-cyclic reversed loading tests were carried out on three PEC braces and one steel brace. Failure modes， hysteretic curves， initial stiffness， bearing capacity， ductility and energy dissipation capacity were studied. The results show that the PEC frame and the PEC brace cooperate well as the two structural seismic lines. And compared to the steel brace， three PEC braces are improved in initial stiffness by 24%-41%， bearing capacity by 29%-36% and energy dissipation capacity by 7%-17%， indicating that PEC braces have excellent seismic performance. Based on the results of the experiment， the nonlinear finite element analysis was carried out by ABAQUS software. The geometric nonlinear behaviors such as local buckling and global buckling are observed in finite element models， and the results such as peak load and yield load are in good agreement with the experimental results （the error is within 10%）. The finite element analysis shows PEC braces yield earlier than the PEC frame， which is the expected failure sequence. Additionally， the stress concentration and local buckling of joint of brace can be effectively inhibited by setting concrete or stiffeners on the web.