Carbon dioxide (CO2) is both a major contributor to the greenhouse effect and an important C1 resource. The synthesis of cyclic carbonates from CO2 and epoxides is an important way of high efficiency and high value utilization. The development of high efficiency catalyst is the focus of research for this reaction. In the past, there were some problems such as poor stability, low catalytic activity and complex preparation of catalyst. Therefore, the aim of this study was to develop phosphine-based catalysts with high catalytic activity, low price and good stability. In this study, a binary composite catalytic system was formed between phosphine oxide compound and metal salt with high stability. By regulating different proportions of metal salts and phosphine oxide compounds, the binary composite catalytic system with different proportions was formed, and the optimal transition metal salts and the optimal ratio were selected (Bu3PO:ZnBr2=2:1, namely [2Bu3PO-ZnBr2]). The influence of catalyst dosage, CO2 pressure, reaction time, and reaction temperature on propylene oxide (PO) conversion and the selectivity of propylene carbonate (PC) was investigated. The results showed that under optimum reaction conditions (130℃, 3 h, 3 MPa), the PO conversion was 92%, the PC selectivity was 99% and the catalyst maintained high catalytic activity after five cycles. Finally, the interactions between the phosphine-based binary composite catalytic system and the reactants were revealed through relevant characterization, and the Lewis acid co-catalytic mechanism was proposed. The phosphine-based binary composite catalytic system proposed in this study provides a new idea for efficient and inexpensive catalyst utilization.