The invention relates to biotoxicity assessment of drilling fluid, in particular to a method for assessing the acute toxicity of the drilling fluid rapidly by utilizing marine microalgae. The method is as follows: carrying out mixed culture on marine microalgae fluid in a steady growth phase and stock fluid of the drilling fluid for 3-8 hours; sampling, and measuring fluorescence emission spectrum to obtain chlorophyll fluorescence intensity of the algae at the position of 678-685nm; taking the fluorescence intensity of the microalgae without the added stock fluid as contrast, and calculating relative mortality rate of the microalgae; and according to the relative mortality rate of the microalgae, utilizing Curveexpert or Microsoft Excel 2003 software to draw a relation curve between the mortality rate of the microalgae and the concentration of the drilling fluid, and determining whether the drilling fluid is toxic or non-toxic according to the national standard on the basis of 7-8h EC50 value. The assessment result to the toxicity of the water-based drilling fluid by utilizing the microalgae method established by the biotoxicity assessment of the drilling fluid is consistent to that of 96h LC50 of artemia, the time is shortened by 88 hours compared with the conventional 96-hour biotoxicity test time, and the field usage can be achieved.