Abstract: Two kinds of water-based phosphorus flame retardants, DAG-50 and DAG-80, and their compound flame retardants were applied to modify the natural jute fibers. A novel type of jute fiber/polyester fiber composites was prepared by the hot-press method using the flame retarded jute fibers and polyester fibers with skin-core structure as raw material. The flame retarded effects and mechanism of these flame retardants were studied by combustion tests, SEM, FTIR, TGA and TGA-FTIR. Meanwhile, the flame retardant formula which is suitable for the industrialization of the jute fiber/polyester fiber composites was also screened out. The results show that DAG-50 presents good flame retardancy, but easily precipitates out on the surface of jute fibers. DAG-80 can coat uniformly on the surface of jute fibers with better flame retardancy, but it has much higher price than DAG-50. The compound flame retardant prepared by DAG-50 and DAG-80 shows excellent flame retardancy towards jute fibers. It is nearly neutral and can avoid the corrosion of equipment and overcome the problem of precipitating out when using DAG-50 alone. Considering cost and flame retardancy, the 55wt% compound flame retardant made up of DAG-50 and DAG-80 with the ratio of 2:1 can achieve flame retardant composites with B1 rank.