NOVELTY - Production of low-carbon alkene gas by transforming methanol includes flowing the gas from the top of demethanizer backflowing pot to expansion machine; cooling; reducing pressure; separating the material flow of the expansion machine; heat exchanging the gaseous phase material and discharging from the zone; sending the liquid material to the front 2 sections of sucking pot of the gas compressor or returning to the demethanizer; and flowing the material at the bottom of the demethanizer to other alkene separating unit to separate ethylene and propylene.    USE - Production of low-carbon alkene gas i.e. ethylene and propylene (claimed).    ADVANTAGE - The method reduces the loss of ethylene in gas at the top of demethanizer, improving yield of ethylene.    DETAILED DESCRIPTION - Production of low-carbon alkene gas by transforming methanol comprises:    (A) compressing raw material gas in 4 sections at 2.5-3.5 MPa; cooling at - 20-40 degrees C; and flowing into demethanizer raw material pot; sending the gaseous phase material flowing from the demethanizer raw material pot to 10-20 layers of theoretical tray of demethanizer for contacting with absorbing agent; sending the liquid material to 15-35 layers of theoretical tray of demethanizer; cooling the gas from the top of the demethanizer at - 45 to - 60 degrees C and providing into backflowing pot of demethanizer; and returning the liquid phase material from the bottom of the demethanizer to the demethanizer;    (B) flowing the gas from the top of the backflowing pot to the expansion machine; cooling the material at - 90 to - 120 degrees C; and reducing pressure to 0.10-0.60 MPa; and    (C) separating the material flow of the expansion machine; heat exchanging the gaseous phase material and discharging from the zone; sending the liquid material to the front 2 sections of sucking pot of the gas compressor or returning to the demethanizer; and flowing the material at the bottom of the demethanizer to other alkene separating unit to separate ethylene and propylene, where the number of theoretical tray is measured from the top of the demethanizer.