Ultra-high performance concrete (UHPC) was designed and prepared by using large substitution level (60%, mass fraction) of limestone and calcined clay instead of cement. Its early hydration behavior and mechanical properties under standard curing and steam curing were studied by compressive strength test, X-ray diffraction (XRD) analysis, isothermal calorimetry analysis and comprehensive thermal analysis. It is found that steam curing significantly improves the loss of compressive strength caused by high cement substitution at 1 d and 3 d, while standard curing brings more excellent compressive strength at 7 d. When the mass ratio of calcined clay to limestone is 2∶1, the strength development of each age is the best. The hydration process is greatly accelerated under the condition of steam curing, and there is an obvious exothermic peak of aluminate phase reaction. Steam curing intensifies the consumption of calcium hydroxide by calcined clay, and the formation of monocarboaluminate is detected only when the mass ratio of calcined clay to limestone is 2∶1, indicating that in the environment of low water to binder ratio, the synergistic effect of early calcined clay and limestone mainly depends on the content of calcined clay.