The invention provides a method for preparing 
biodiesel from microalgae. The method comprises the following steps of: a. changing collected microalgae from the wet 
algae into 
algae block or 
algae powder; b. mixing a catalyst into low-carbon 
alcohol, directly adding the algae block or the algae 
powder obtained by the step a, and carrying out ester exchange reaction to prepare the 
biodiesel; c. after the reaction is stopped, adding an 
organic solvent for extracting reaction solution which is divided into 
organic solvent phase used for extraction, low-carbon 
alcohol water-adding phase and algae mud; and d. after the extraction is completed, collecting the 
organic solvent phase used for extraction, removing the organic 
solvent by 
distillation, and obtaining oily liquid, namely crude products of the 
biodiesel. After preparing the biodiesel, the steps that the low-carbon 
alcohol water-adding phase is used circularly after dewatered by a 
solid drying agent, and the produced algae mud is used for producing 
biogas by biological 
fermentation are added. The invention completes the oil extraction of microalgae and 
biodiesel production by one step, simplifies the technique steps and equipment, saves cost; the sulphuric acid and low-carbon alcohol used for production can be recovered, then dewatered by 
drying, then utilized repeatedly, the cost is reduced, the final product is neutral, does not need washing and reduces the 
downstream processing pressure; and simultaneously, 
chlorophyll is mainly concentrated in the low-carbon alcohol, reduces the interference of the 
chlorophyll to the color of the product and does not need the step of decoloring.