The convertion of acid oil to biodiesel by use of immobilized Candida lipase absorbed on textile cloth was studied in a fixed bed reactor,which can not only reduce the environmental pollution of acid oil, but also produce a substitute for petroleum diesel. The acid oil mixed with methanol was pumped into three fixed bed reactors in series, and the methanol was added with the molar flow rate same as the acid oil in each reactor. The effects of enzyme content, solvent content, water content, flow rate of reactant and temperature on the enzymatic reaction were analyzed. The result of orthogonal experiments indicates that the optimal transesterification can be performed under the following conditions: immobilized lipase content in acid oil, 20%; hexane content in acid oil, 10%; water content in acid oil, 10%, reaction temperature, 50℃; and flow rate of reactant, 0.08g/min. Under these conditions, the FAME content of 90.18% in the product is obtained. The immobilized lipase can be reused with relatively stable activity after glycerol being removed from the surface. By refining, most of the chemical and physical properties of biodiesel will meet the American and Germany biodiesel standards and exceed the Chinese standard of 0＃ petroleum diesel except for carbon residue, density and kinematic viscosity.