NOVELTY - A composite catalyst comprises (wt.%): acetic acid (10-20), methylsulfonic acid (10-20), benzene sulfonic acid (15-30), oxalic acid (15-30) and sulfuric acid (0-50).    USE - Composite catalyst for producing biodiesel.    ADVANTAGE - The catalyst reduces three-waste discharge, and has high product index, wide raw material adaptability, high product conversion rate and low production cost.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for production of the catalyst which comprises:    (A) adding methanol A into no.1 reaction kettle, and opening to stir at 60-70 at revolutions/minute (rpm);    (B) slowly adding benzene sulfonic acid (6 wt.% of methanol A) and oxalic acid, and keeping temperature in kettle at 20-30 degrees C, where temperature is not high enough to open heating system to heat;    (C) taking benzene sulfonic acid (4 wt.% of methanol A) and acetic acid, maintaining at 25 degrees C, where benzene sulfonic acid and acetic are liquid, adding into reaction kettle to uniformly stir, and obtaining no.1 catalyst liquid;    (D) adding methanol B into no.2 reaction kettle, opening to stir at 70-80 rpm, slowly adding 20% of purified sulfonic acid (20 wt.% of methanol B), and obtaining assistant catalyst F;    (E) respectively uniformly preparing no.1 catalyst and assistant catalyst F in ratios of 1:0, 2:1 and 1:1, obtaining no.1. no.2 and no.3 catalyst; and    (F) diluting no.1 catalyst, no.2 catalyst and no.3 catalyst for 2-5 times by methanol , preparing biodiesel in ratio of 1:3, where catalysts above can respectively used to different raw materials.    Fat with low acid value and low water impurity applies to no.2 catalyst. Fat with middle value and water impurity applies to no.2 catalyst. Fat raw material with high acid value and high water impurity applies to no.3 catalyst. Low acid value fat raw material comprises potassium hydroxide (less than 30 mg). Middle acid value fat raw material comprises potassium hydroxide (30-80 mg). High acid value fat raw material comprises potassium hydroxide (more than 80 mg).