NOVELTY - A gas phase process for circularly producing biodiesel comprises: (a) sending biodiesel material in a reaction kettle, heating material; (b) sending methanol steam from the methanol generator in the kettle; carrying out grease exchange reaction; (c) making one part of the methanol steam with water in the material enter in a cooler from the upper part of the reaction kettle, making methanol steam pass through a methanol recycling tower, separating water dissolved in methanol, obtaining 99.9% of fine methanol, refluxing; and (d) obtaining 98% of crude fatty acid methyl ester and distilling.    USE - The gas phase process and the device are useful for circularly producing biodiesel (all claimed).    ADVANTAGE - The gas phase process and the device improves the temperature in the reaction kettle to shorten esterification time, improve the oil yield of the biodiesel, reduce 50% of methanol consumption, and greatly improve production efficiency.    DETAILED DESCRIPTION - A gas phase process for circularly producing biodiesel comprises: (a) sending biodiesel material in a reaction kettle, heating up the material while adding catalyst having weight that is 5-7% of that of the material in the kettle, sufficiently agitating; where the catalyst is 98% of sulfuric acid; (b) sending methanol steam from the methanol generator under the liquid plane in the kettle when the temperature in the kettle is increased to 90-95 degrees C; where the dosage of the methanol steam is calculated by liquid methanol; and the dosage of the liquid methanol is 8-10% of the weight of the material; carrying out grease exchange reaction to the methanol steam and the material for 45 minutes to 1 hour; (c) making one part of the methanol steam with water in the material enter in a cooler from the upper part of the reaction kettle, making the methanol steam pass through a methanol recycling tower, separating water dissolved in the methanol, obtaining 99.9% of fine methanol, refluxing the methanol into the methanol generator to be circularly used; and (d) obtaining 98% of crude fatty acid methyl ester by rinsing after the esterification reaction, distilling with short range molecule distiller, and obtaining 95% of fine fatty acid methyl ester that is the biodiesel. An INDEPENDENT CLAIM is a device for achieving the gas phase process for circularly producing biodiesel comprising a reaction kettle; where the lower part of the reaction kettle is connected with the upper part of the methanol reactor; a cooler is connected at the upper part of the reaction kettle; the cooler is connected with a methanol recycling tower by a storage tank; the methanol recycling tower is connected with a methanol storage tank by a pump; and the methanol storage tank and the lower part of the methanol reactor are connected.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the device for achieving the gas phase process for circularly producing biodiesel.