Abstract: The hydration exothermic characteristics of fly ash micro beads and superfine slag powder when they are mixed alone or in combination at a water-binder ratio of 0.16 were studied by isothermal calorimeter. The influence on the compressive strength of paste were discussed, and two low-heat cementitious systems were proposed. The results showed that the 56 d compressive strength could still reach 157.3-176.5 MPa when the total dosage of micro beads and super slag powder was 50%-65%, which was not lower than that of pure cement system; compared with superfine slag powder, micro beads had more prominent peak shaving and heat reduction capacity. When the two were mixed together, the exothermic rate and total hydration heat in the early induction period and acceleration period could be further reduced as a whole. The second exothermic peak could be as low as 48.0% of the straight cement. When the content of micro beads was more than 35%, the superposed effect of superfine powder made the exothermic rate in the acceleration period before 8.9 h higher than that of straight cement. The X-ray diffraction and thermogravimetry analysis showed that the superposed effect did not accelerate the formation of early ettringite, but it promoted the early activation of pozzolanic activity to a certain extent after 3 days, and generated more low alkaline hydrated calcium silicate and gismondine; the fiber-free ultra-high performance concrete with 56 d compressive strength of 150-160 MPa was prepared by using the proposed low-heat cementitious system. Their adiabatic temperature rise ΔT was 37.4-41.2℃, and the minimum cement dosage was 35%, which could provide a reference for the preparation of low-heat ultra-high performance concrete above 150 MPa.