the invention discloses a preparing method of biological diesel based on p-toluenesulfonic acid as catalyst, which is characterized by the following: adopting high-acid value plant oil at 5-200mgKOH/g as raw material; setting the final reacting temperature at 60-75 deg.c and catalyst quantity at 0.5-3% corresponding to high-acid value plant oil; making the quality rate of alcohol and oil at 1:1-1:3; reacting for 2-4 h; proceeding film evaporation disposal when the acid value is 5-100mgKOH/g; proceeding supplementing disposal of ester exchange for triglyceride. |