NOVELTY - Preparation of biodiesel by using ring pipe reactor comprises mixing raw material oil with low alcohol based on alcohol-oil, throwing it into ring pipe reactor, regulating reaction pressure, leading heat conductive oil into jacket of reactor as heat source, regulating flow of heat conductive oil, controlling reaction temperature while materials flow circularly, regulating recycle ratio of medium in regulation pipe, in full mixing condition, in the action of catalyst, plant oil and low alcohol take ester exchange reaction to obtain biodiesel.    USE - The process is useful for preparing biodiesel (claimed).    DETAILED DESCRIPTION - Preparation of biodiesel by using ring pipe reactor comprises mixing raw material oil with low alcohol based on alcohol-oil, where mol ratio is 4-16, catalyst is sodium hydroxide (NaOH) or potassium hydroxide (KOH), and concentration is 200-50000ppm of plant oil weight, throwing it into ring pipe reactor by using metering pump, regulating reaction pressure at 0.2-10.0MPaG, leading heat conductive oil into jacket of reactor as heat source, regulating the flow of heat conductive oil, making reaction temperature be controlled at 60-300 degrees C while materials flow circularly, the residence time of reaction materials in reactor is 10-120 minutes, regulating the recycle ratio of medium in regulation pipe at 25-250 by regulating the flow of recycle pump, in full mixing condition, in the action of catalyst, plant oil and low alcohol take ester exchange reaction to obtain biodiesel. Reaction pipe 8 is composed of one or many linear reaction pipe 8-4, the external of linear reaction pipe 8-4 is set with jacket 8-5, linear reaction pipes 8-4 are connected by ring pipes 8-3, the first linear reaction pipe and the final linear reaction pipe are connected by connection pipe to form close loop, recycle ring pump 7 is set on the close loop of ring pipe reactor 8.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of preparing biodiesel by using ring pipe reactor.