The invention relates to an alginate dressing for treating bedsore and a preparing method thereof. The dressing has a double-layer composite structure, namely the dressing comprises a base layer and a surface layer. The base layer is an alginate composite film layer. The surface layer is a natural rubber film layer. The preparing method includes following steps of: A) preparing a shark collagen solution, B) preparing a coaxial electrospinning outer layer solution, C) preparing a coaxial electrospinning core layer solution, D) spinning, E) crosslinking, F) dipping, G) preparing an epoxy natural rubber film, and H) compositing. The dressing has good biocompatibility, and has controllable growth regulation, good hygroscopicity, good anti-inflammation performance and good antibacterial performance of synthetic biological materials. The dressing is capable of improving local blood supply of wounds, promoting tissue growth and accelerating wound healing, shows good multifunctional properties, has good porosity, soft and comfort hand feeling, good compliance and using convenience, and can be used in the field of modern biomedicine, especially in the field of bedsore treatment and nursing, and the like.