To study the time-varying characteristics and models of mechanical properties of mechanism sand concrete mixed with the fly ash and the slag (referred to as multi-mixed concrete) under high-temperature steam curing, combined with a high-temperature steam curing multi-mixed concrete precast beams of a highway project, steam curing-standard curing, steam curing-natural curing and standard curing modes were designed to test compressive strength, splitting tensile strength and elastic modulus of multi-mixed concrete at different periods. The relationship between compressive strength and splitting tensile strength, elastic modulus were explored. The results show that a suitable steam curing method can accelerate the growth of compressive strength, splitting tensile strength and elastic modulus of multi-mixed concrete, with the most significant effect on the development of compressive strength, and the mechanical properties don’t retract in the later stage. During the constant temperature stage of steam curing, the mechanical properties of concrete have the highest growth rate, followed by the heating stage, and the mechanical properties have the lowest growth rate during the cooling stage. Under steam-standard curing mode, time-varying model of compressive strength of multi-mixed concrete can take logarithm or logarithm and power function composite function, and the time-varying model of splitting tensile strength and elastic modulus can take power function.