Provided are a process and a device for preparing environmentally-friendly insulating gas perfluoroalkylnitrile. The device comprises a first tank reactor, a second tank reactor, and a fixed bed reactor. A condensing and collecting device is provided at the top of the first tank reactor, a collecting and guiding channel at the bottom of the condensing and collecting device is connected to an inlet of a storage tank, and an outlet of the storage tank is connected to an inlet of the second tank reactor. The second tank reactor is connected to a perfluoroolefin tank. An outlet of the second tank reactor is connected to a storage gasification tank. The storage gasification tank is connected to a carrier gas tank, and an outlet of the storage gasification tank is connected to an inlet of the fixed bed reactor. The present invention starts from cyanuric chloride and perfluoropropylene, and uses two tank reactors and a fixed bed reactor to realize production with high atom economy. The preparation method has simple reaction conditions, high atom economy, low cost, and is continuous and enables scale production.