The improvement of rice husk ash(RHA) on the strength of cement ground limestone pastes was studied. The hydration degree and products of cement ground limestone RHA composite pastes were determined by thermogravimetry and XRD, and the action mechanism was analyzed. The results show that the compressive strength of composite pastes increases at first and then decreases with increasing content of RHA. When RHA content is 10%, the compressive strength of composite pastes is the highest. The compressive strength of 10% RHA and 10% ground limestone composite pastes is 821%, 1843% and 175% higher than that of pure cement pastes at 3, 7, 28d, respectively. 15% RHA is helpful to the extent of compressive strength increasing with age. RHA has a certain filling effect, activation effect and internal curing effect. When the content is less than 10%, the filling effect and activation effect of RHA play a leading role, which can accelerate the hydration of C3S, and participate in the secondary hydration, and improve the compressive strength of the composite pastes. When RHA content increases to 15%, excessive RHA absorbs a large amount of water, reduces the hydration degree of cement, and leads to the low compressive strength of composite pastes at early ages. The water in RHA is gradually released with the increase of ages. Due to the internal curing effect, RHA promotes the hydration and participates in the secondary hydration of cement. Thus, the compressive strength of pastes is improved.