Abstract: It is an important technology for slurry recycling by sand removal in flowback slurry based on chemical principles and also an active measure to promote tunnelling efficiency and environmental protection. By analyzing the molecular structure characteristics of partially hydrolyzed polyacrylamide (HPAM), it reveals that the ionized group and linear soft chain can selectively capture and coagulate tiny inertia rock debris while the clay is still dispersed in the slurry system. The effect of HPAM selective flocculation is verified by experiments like vertical tube pressure test. And analysis and selection are conducted to this kind of chemical agent by taking molecular weight, hydrolysis degree and fortified concentration as the main controlling factors, providing a new method of sand removal for the slurry used in the Yangtze River-crossing shield tunnel of Wuhan metro line 7.