The invention relates to a method of thick mash alcohol fermentation by using fresh cassava, comprising the following steps: preparing fresh cassava which is pulverized for the first time into pulp; heating, liquefying and filtering; washing with hot water and filtering slag out; recycling residual sugar in a filtering slag, wherein filtrate obtained after filtering can be used for the first pulverization of the fresh cassava, the first filtrate can be used as pulverizing liquid in second pulverization of the fresh cassava. Slurry obtained after pulverization is used as row material of the thick mash alcohol fermentation, and the initial total sugar concentration reaches to 23.5-24.5 (g/100mL); and after the procedures of liquefying, sterilization, saccharifying, fermentation, and the like, the obtained final fermentation mature raw spirit part reaches to more than 13.0(v/v). The invention completely uses the fresh cassava as raw material to ferment thick mash to produce alcohol, thereby effectively improving the initial total sugar concentration in the process of the thick mash alcohol fermentation by using the cassava, and solving the problems of big slurry viscosity, bad flowability, low mass transfer efficiency, and the like in the fermentation process. The invention has the advantages of simple process, energy saving, low cost, low waste material production, and the like,and is used for developing circular economy.