Fused iron catalyst is an active and classical catalyst both for ammonia synthesis and Fischer-Tropsch synthesis which are two typical industrial processes. As it is of significance to catalysis science as well as its importance to industry, fused iron catalyst is an ideal model system for heterogeneous catalysis research. However, the precursors of fused iron catalysts were confined to Fe3O4 in the most studies. The discovery of wustite (Fe1-xO) based catalyst is a key innovation for fused iron catalyst which necessitates the further research to obtain deeper insight into this novel catalyst. This paper summarized three major scientific issues involved in the development of wustite based catalyst, namely, the physicochemical properties, characteristics of wustite, as well as the mechanism leading to the high activity. Based on this novel catalyst, new concepts and opinions were proposed, and the insights and principles obtained are as follow. Hump like activity trend of reduced catalyst was first obtained with varying of Fe2+/Fe3+ in the catalyst precursor. Molecular ratio of iron oxides, single phase theory and structure of active sites were suggested to elucidate the high activity of wustite based catalyst. In addition, new concepts and opinions such as the disproportionation of metastable wustite and its inhibition, matching of promoters with the structural characteristics of wustite, synergy between acidic site and basic site on the surface of catalyst, role of Al2O3 in the re-structuring of catalyst surface, mechanism of competitive reduction, strong H-2 adsorption, active sites, micro-structure of alpha-Fe and the essence of high activity were proposed to bring light to the structure-activity relationship forming the theory system with single phase theory as the kernel of wustite catalyst. These regularities and the theory system significantly promoted and enriched the scientific knowledge of fused iron catalyst. It is unexpected that a knowledge based improvement of a catalyst would have a strong effect on other fields of catalysis research.