In this paper, super absorbent polymer (SAP) is used as anti-washout admixtures (AWA) to investigate the influence of the swelling property, salt resistance swelling property and chemical interaction of SAP on the washout of underwater cement grout. Herein, a new villus-like nanocomposite hydrogel (V-NC gel), one self-synthesized new SAP, were used to fabricate underwater cement grout with ultra-stable anti-washout performance. Incorporating V-NC gel can significantly reduce underwater cement grout washout loss from 91.05% to 62.22%, which was much better than those of adding commercial water absorbent polyacrylate sodium (PAAS). The villus-like structure of V-NC gel ensured super water absorbing performance in the cement environment, leading to strengthening the anti-washout performance. Further, the –COO group (from V-NC gel) could chelate with Ca (released from cement hydration) to form a cross-linking network, being conducive to gluing the cement particles, thereby enhancing the anti-washout performance of the cement grout.