The invention provides a biological acid adjustment method applied in thick mash alcoholic fermentation of cassava, belonging to the technical field of alcohol production through a fermentation method. According to the method, cassava slices are sequentially subjected to pretreatment, cooking and liquefaction, initial saccharifying, combined inoculation, concurrence of lactic acid fermentation and alcoholic fermentation, and saccharifying under alcoholic fermentation, the pH value of krausen is adjusted by lactic acid generated by growth and metabolism of lactic acid bacteria, through the staged temperature controlled fermentation and the growth competition effect of brewer's yeast as well as the competitive inhibition effect of alcoholic fermentation thereof, the lactic acid fermentation degree of the lactic acid bacteria can be effectively controlled, the purpose of adjusting the pH value of krausen is achieved, and the pH value of the krausen achieves the optimum pH value ranges of the saccharifying enzyme and alcoholic fermentation, so that the saccharifying enzyme has the relatively high activity in the fermentation process, and the saccharifying under alcoholic fermentation is carried out efficiently. The method has the characteristics of high production safety, simplicity and feasibility in operation, energy conservation and consumption reduction, low production cost, and the like, and is suitable for being adopted in the actual production of thick mash alcoholic fermentation of cassava.