Achieving desired yield stress is crucial for various construction processes, especially in the emerging 3D concrete printing. In this paper, we present the possibility of enhancing yield stress of cement paste through in-situ polymerization and uncover the mechanisms of in-situ polymerized polymers. We investigate the effects of two types of in-situ polymerized polymer, sodium polyacrylate (PAAS) and polyacrylamide (PAM), on the yield stress, interstitial solution viscosity, and flocculated structure, as well as their adsorption behaviors on the surface of cement particles. The results show that the yield stresses of both in-situ polymerization modified cement pastes are raised considerably at high polymer concentration, which can be attributed to the dual roles of generated polymers. On the one hand, the absorbing polymers can bridge cement particles together, resulting in an augmentation in the interparticle forces. On the other hand, the non-adsorbing polymers remaining in the interstitial solution interact with each other and create a robust polymer network that can effectively withstand the external stresses. The former mechanism emerges as the dominant contributor to the enhanced yield stress of cement paste modified with in-situ polymerized PAAS, attributed to the Freundlich multilayer adsorption behavior of PAAS amplifying the bridging effect of polymers. While in-situ polymerized PAM predominantly assume the latter role, largely due to the remarkable length of their chain structure and the abundant residual polymers in the interstitial solution, which greatly intensities the interaction between polymers.