The invention relates to a heat-shrinkable pipe, in particular to a high-pressure flame retardant single-wall heat-shrinkable pipe and a preparation method thereof. The high-pressure flame retardant single-wall heat-shrinkable pipe comprises wall materials. The wall materials comprise the following components in parts by mass: 90 parts of polyolefin mixed resin, 10 parts of auxiliary agent and 30 parts of flame retardant agent. The preparation method comprises the steps of preparing, extruding, irradiating, expanding and moulding inner wall materials of a flame retardant single-wall heat-shrinkable pipe. Compared with a traditional shrinkable pipe, the invention has the technical characteristic of flame retardance of the wall materials, can be applied to high voltage of 35kV, has all use performances of a common heat-shrinkable pipe and the performances of water prevention, seal, moisture prevention, corrosion prevention, and the like, completely meets the ROHS environmental-friendly requirement of nations, such as European Union, and the like and has the advantages of low-temperature shrinkage, stable expansion for 3 times to 4times, low longitudinal shrinkage rate, and the like.