The invention relates to a 
microorganism water purification agent and a preparation method thereof. The preparation method comprises following specific steps of: inoculating the agent onto a 
cane molasses medium, and fermenting for 24-120 hours at 25-35DEG C so as to cultivate the agent. The method for purifying the 
water quality of 
aquaculture water by the prepared 
microorganism water purification agent comprises the step of adding 15-18ml of the agent prepared by the method into one cube of 
aquaculture water. According to the method, the 
cane molasses medium is used for cultivating the agent, so that the amount of viable 
bacteria can be increased to the greatest extent, the amount of viable 
bacteria of 
bacillus subtilis, which plays a main role in purifying the 
water body, can be increased, and the purification effect of the 
aquaculture water is better; for the 
fermentation time, the amount of viable 
bacteria of the agent can be improved, furthermore, and the agent holds certain amount of 
sugar so as to provide 
nutrient for the bacteria, thus being beneficial to guaranteeing the quality; the agent prepared by the method can effectively solve the problem of damage and 
eutrophication of the quality of the aquaculture water, thus improving the quantity and quality of aquatic products.