The effects of hydroxypropyl methylcellulose (HPMC) on the rheology and viscoelasticity of cement-limestone paste were evaluated, as was the mechanism of HPMC from the viewpoint of zeta potential and adsorption amount. The results revealed that the greater the content of HPMC or the viscosity of its aqueous solution, the lower the fluidity of the composite paste and therefore the higher the rheological parameters. The relation between torque and rotational velocity of cement-limestone paste is linear; nevertheless, the shear thickening degree of paste increased following the addition of HPMC, demonstrating typical viscoelastic properties. The addition of HPMC prolonged the induction duration and delayed the emergence of the exothermic peak of hydration, decreased zeta potential and conductivity, and increased the propensity of suspension particles to agglomerate. The adsorption amount of polycarboxylate superplasticizer (PCE) decreased with increasing aqueous solution viscosity and HPMC concentration due to the winding effect of HPMC’s high molecular chain structure and the competing adsorption of HPMC and PCE.