Abstract: The action mechanism of admixtures on flocculation and separation characteristics of waste slurry in slur? ry shield construction is still unclear. To figure out this, waste slurry with different gradients of bentonite and CMC(Carboxymethyl cellulose sodium) are prepared, so as to study the influence of residual admixtures on the flocculation and separation characteristics of waste slurry and the action mechanism. Research shows that the presence of bentonite and CMC deteriorates the dewatering effect of slurry, and increases the specific filtration resistance by 1~2 orders of magnitude; Bentonite and CMC affect the water form of soil particles by changing their electrical characteristics and by adsorption and bridging, deteriorating the dehydration effect; Polyaluminium chloride (PAC), which has the function of electrical neutralization and compressing polymer molecular chains, has achieved better results in waste slurry treatment. Based on the research results, the countermeasure for the preparation of shield slurry and the treatment of waste slurry is proposed.