The invention provides a preparation method of polyacrylonitrile oxidized fiber felt. The method is characterized in that firstly, polyacrylonitrile short fiber is made into felt through carding and net forming and needling; then, aqueous emulsion of polyorganosiloxane and guanidine compounds is used for performing chemical modification treatment on the felt; finally, drying and oxidation treatment are performed to prepare the polyacrylonitrile oxidized fiber felt. The method has the advantages that the operation is simple; the production efficiency is high; the production cost is low. The prepared polyacrylonitrile oxidized fiber felt has excellent flame retardant property; the limited oxygen index can reach 40 percent or higher; the shrinkage distortion is small; the flexibility is high.