The coupling effects of steel slag powder (SSP) and electromagnetic waves on hydration kinetics and microstructure evolution of ultra low water/binder ratio cementitious materials (ULWC-SSP) is clarified in this research. More precisely, a dense matrix skeleton with appropriate content of SSP is designed first of all by the D-optimal design (DOD). After that, the ULWC-SSP samples are subjected to the electromagnetic waves environment for 0–4 min. Finally, the coupling effects of SSP and electromagnetic waves on the macroscopic and microscopic performances are evaluated. The obtained results show that the amount of early C–S–H gels of samples cured by electromagnetic waves for 1–4 min increases from 28.63% to 31.71%. At the same time, SSP can be used as a local heat source and electromagnetic shield to improve the interfacial transition zone (ITZ) under the treatment of electromagnetic waves. Therefore, based on an optimal curing regime, the macro and micro properties of the developed sustainable ultra low water/binder ratio cementitious materials can be advanced.