NOVELTY - Preparation of biodiesel with methylalated naphthalene sulfonic acid as catalyst, comprises adding oil, low-carbon alcohol and methylalated naphthalene sulfonic acid catalyst, in three-neck container; rising temperature; filtering reacted materials, filtering out catalyst and filtrate; dividing filtrate and removing under-layer glycerine; adding sodium bicarbonate (NaHCO3) solution; washing upper-layer mixture till it becomes neutral; dividing out upper-layer organic phase; drying organic phase; filtering; reducing pressure to distil and obtaining final product.    USE - The process is useful for preparing biodiesel (claimed).    ADVANTAGE - Preparation of biodiesel with methylalated naphthalene sulfonic acid as catalyst has high yield, cheap catalyst, small dosage of catalyst, mild reaction condition, short reaction time; the catalyst can be used again and again; it is also environment friendly without side reaction, such as saponification; and it is no need to pre-treat raw materials.    DETAILED DESCRIPTION - Preparation of biodiesel with naphthalene sulfonic acid methylal as catalyst, comprises adding oil, low-carbon alcohol and methylalated naphthalene sulfonic acid catalyst, in three-neck container with electromagnetic mixer, temperature meter and reflow cooling pipe; rising temperature at 50-110 degrees C; filtering the reacted materials, filtering out catalyst and filtrate; dividing the filtrate and removing under-layer glycerine; adding NaHCO3 solution with quality ratio of 5-10% in upper-layer mixture, dosage of the NaHCO3 is as 2-6 times as that of upper-layer mixture; washing the upper-layer mixture till it becomes neutral material; dividing out upper-layer organic phase by tap funnel; drying the organic phase by anhydrous magnesium sulfate (MgSO4); filtering it to remove desiccant; reducing pressure to distil and obtaining final product, that is transparent festucine biodiesel. Quality ratio of oil, low-carbon alcohol and methylalated naphthalene sulfonic acid is 1:0.1-0.5:0.04-0.1. Oil is plant oil, animal oil, waste oil or hogwash oil; and low-carbon alcohol is methanol, ethanol, propanol or butyl alcohol.