Addition of the calcium silicate hydrate（C S H) powder to cement paste was aimed to investigate its influence on the cement hydration kinetics and mechanism. Hydration rate and hydration heat were measured by TAM air calorimetry. The contact angle of the C S H powder was evaluated by the permeating height; kinetic parameters, such as reaction rate(K), hydration degree(α), and reaction order(n) were evaluated according to the analysis of the hydration kinetics. Additionally, the influence of the C S H powder on hydration mechanism of cement was discussed according to the differential curve. Results indicate that the time to reach the rate peak and the total heat of cement hydration decrease with the addition of C S H powder, and nucleation barrier is reduced as well, which is attributed to the seeding effect of the C S H additives by providing new nucleation sites. Given a certain amount of C S H（1% to 3% by mass), hydration mechanism is described as nucleation and crystal growth（NG) interaction at phase boundaries(I)diffusion(D). Addition of C S H powder can accelerate the NG process, and has inhibition effect on the I process. It has no influence on the D process.