In order to research the rapid mud-water dehydration properties of waste slurry generated by the construction of subway, to improve the efficiency of the separation, and to reduce the cost of slurry treatment, chemical flocculation method was used in this research to analyze the effects of flocculant type, dosing quantity and agitation on flocculation efficiency of clay slurry. The results showed that, the flocculation effects of slurry was best when adding 0.02-0.03 g of polyacrylamide, 0.03g of coal ash at stirring speed of 400-450 r/min and stirring time of 240-270 seconds in 50 mL slurry. The treatment method can effectively reduce the amounts of slurry treatment, shorten the slurry treatment cycle, save the cost and produce more social benefits.