Selective laser melting (SLM) was applied to fabricate high-strength and ductile reduced activation ferritic/martensitic (RAFM) steel via modification of heterogeneous microstructure that consists of domains of fine and coarse grains as well as lath martensites, but with different grain morphology in respond to the variation in the scanning strategy and resultant thermal history. The SLM-built RAFM steel exhibited an enhanced strength-ductility synergy with high yield strength of 967 MPa and high elongation of 15.1% under a meander scanning strategy, surpassing the previously reported counterparts, which was ascribed to the refined microstructure and improved strain hardening. In contrast, an island scanning strategy deteriorated the ductility down to 3.8% arising from the oriented alignment of columnar coarse grains around island, which initiated micro-voids and cracks during deformation and finally gave rise to a premature failure. This work proposed the microstructural design principles for additively manufacturing high-performance RAFM steels and is expected to facilitate their application in the fusion reactor.