NOVELTY - Synthesis of biodiesel by using p-toluenesulfonic acid formaldehyde condensate as catalyst and by ester exchange process, comprises taking p-toluenesulfonic acid formaldehyde condensate as catalyst, filtering catalyst, separating, alkali washing, and distilling to obtain biodiesel, where it specifically comprises adding oil, low-carbon alcohol and p-toluenesulfonic acid formaldehyde condensate with ratio of 1:0.1-0.5:0.04-0.1 in three-necked container, and distilling to obtain biodiesel. The oil is plant oil or waste oils, and the low-carbon alcohol is methanol, propanol, or butanol.    USE - The process is useful for synthesizing biodiesel (claimed).    ADVANTAGE - The invention has high yield, cheap cost of catalyst, few catalyst dosage, mild reaction condition, short reaction time, repeatedly used catalyst, no environment pollution, no side effects like saponification, and low raw material requirements.    DETAILED DESCRIPTION - Synthesis of biodiesel by using p-toluenesulfonic acid formaldehyde condensate as catalyst and by ester exchange process, comprises taking p-toluenesulfonic acid formaldehyde condensate as catalyst, filtering catalyst out of the reaction mixture after reaction is over, separating, washing, alkali washing, and distilling residue to obtain biodiesel, where it specifically comprises adding oil, low-carbon alcohol and catalyst p-toluenesulfonic acid formaldehyde condensate in three-necked container equipped with electromagnetic stirrer, thermometer and reflux condensation tube, heating to reflux, stirring for 1-10 hour, controlling temperature at 50-110 degrees C, filtering the reacted material to filter out catalyst and residue, separating lower glycerine out of the residue, adding sodium bicarbonate (NaHCO3) solution of 5-10% having dosage of 2-6 times of quality of upper mixture in upper mixture, washing until it is neutral, separating out upper organic phase with separating funnel, drying organic phase with water-free magnesium sulfate (MgSO4), decompression distilling to obtain the pale yellow transparent biodiesel, where the quality ratio of the added oil, low-carbon alcohol, catalyst p-toluenesulfonic acid formaldehyde condensate is 1:0.1-0.5:0.04-0.1. The oil is plant oil, animal fat, or waste oils in production process, and the low-carbon alcohol is methanol, ethanol, propanol, or butanol.