The invention discloses a flexible biodegradable polytrimethylene carbonate composite film and a preparation method thereof. The composite film is formed by blowing a film after melting and mixing polytrimethylene carbonate, thermoplastic polyurethane elastomers, a bulking agent, antioxygen, a heat resistant stabilizing agent, a slipping agent, an antiblocking agent, a filling agent, a biodegradation accelerating agent and a light decomposition agent. In the invention, PPC and elastomers TPU are effectively compounded, thereby solving the problems of brittleness and low-temperature viscous flow of materials; the processing temperature of unsealed PPC is expanded to 170 DEG C, and the thermal degradation temperature is improved to 210 DEG C. Meanwhile, the composite film has good mechanical property and film blowing stability, can be applied to the fields of soft film materials, such as disposable shopping bags, land films, preservative films, and the like, and opens a new approach forthe popularization and the application of PPC.