The invention belongs to the field of preparation of CSM (chlorosulfonated polyethylene), and particularly relates to a method for preparing oil-extended CSM through a gas-solid reaction. The method comprises steps as follows: high-Mooney-viscosity CPE (chlorinated polyethylene) powder and an anti-bonding agent are added to a reactor, stirring is started, the inside of the reactor is vacuumized, the reactor is filled with hot nitrogen and then decompressed, the process is repeated for multiple times, an ultraviolet lamp is turned on for irradiation, chlorine and sulfur dioxide mixed gas is fed into the reactor, a reaction product is transferred to purified water to be washed repeatedly until the pH of the washing water is higher than 6 after the reaction, then a material is dehydrated and dried, high-Mooney-viscosity CSM powder is obtained, the high-Mooney-viscosity CSM powder and rubber processing oil are put into a high-speed stirrer in proportion, a stabilizer is heated, mixing and forming are performed by the aid of conventional rubber mixing and forming equipment, and the oil-extended CSM product is obtained. With the adoption of the method, problems that a main raw material, namely, low-Mooney-viscosity CPE, is difficult to produce, the CSM product is not high in tensile strength, the production cost is high and the like in the process for producing the CSM with an existing gas-solid method can be solved.