This paper studies the mechanism of anti-washout performance of hydroxypropyl methylcellulose (HPMC) and polyacrylamide (PAM) in seawater-mixed cement paste. Using seawater as concrete mixing water can reduce the consumption of freshwater resources. The ionic environment of seawater can promote hydrogen bonds formation of HPMC, and PAM. The viscosity of the solution increased with the numbers of hydrogen bonds, thus, the viscosity of seawater-mixed cement paste increased by incorporating HPMC and PAM. The viscosity of admixtures (AWA) solution is a major determinant of the viscosity and washout resistance of cement paste. Incorporating HPMC can significantly reduce underwater seawater-mixed cement paste washout loss from 59.8 to 0%, while incorporating PAM can reduce it from 59.8 to 2.8%. Compared with PAM, HPMC has better anti-washout effect because of its high viscosity in seawater-mixed solution. This research reports an effective method to improve the washout resistance performance of seawater-mixed cement paste through polymer doping and illustrates the feasibility of using seawater instead of freshwater applied in the underwater anti-washout cement paste.