NOVELTY - An external cycle fermentation technique comprises: (a) feeding: naturally flowing into a ferment cylinder of the device from a discharge port of a dry type ferment device by a charged material; (b) heating, insulating heat, and fermenting: detecting the temperature in the ferment cylinder by a temperature sensor arranged on the ferment cylinder of the device; and (c) discharging: continuously fermenting the material in the ferment cylinder to generate the marsh gas, the biogas residue, and the biogas slurry in the ferment cylinder; and discharging the material from the respective outlets.    USE - The external cycle fermentation technique is useful for crop straws.    DETAILED DESCRIPTION - An external cycle fermentation technique comprises: (a) feeding: naturally flowing into a ferment cylinder of the device from a discharge port of a dry type ferment device by a charged material; (b) heating, insulating heat, and fermenting: detecting the temperature in the ferment cylinder by a temperature sensor arranged on the ferment cylinder of the device; where when the temperature is lower than a design temperature, the control system of the device is automatically started; a circulation pump pumps the material in the ferment cylinder to a heat exchange device; and after the temperature is increased, the material is returned to the ferment cylinder by the circulation pump; and circulating to stir the material in the ferment cylinder; where the design temperature is reached; and (c) discharging: continuously fermenting the material in the ferment cylinder to generate the marsh gas, the biogas residue, and the biogas slurry in the ferment cylinder; and discharging the material from the respective outlets. An INDEPENDENT CLAIM is an external cycle fermentation technique device, comprising a ferment cylinder, a heat exchange device, a feeding port, an inner film, an outer film, a safety valve, a discharge valve, a fan, a temperature sensor, an electric valve, a circulation pump, a stop valve, and a control system; where one side out of the ferment cylinder is set with a heat exchanger; the other side of a feeding valve is set with a marsh gas outlet valve and the discharge valve; the top of the cylinder is set with the inner and outer films in turn; the upper part of the outer film is set with the safety valve; an inflation cavity is arranged between the inner and outer films; the fan and a pipeline are connected with the inflation cavity; the lower part of the cylinder is set with the temperature sensor; and the control system is arranged outside the ferment cylinder and electrically connected with the temperature sensor and the heat exchanger.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the external cycle fermentation technique device.