The relationships among catalytic activity, reduction performance, and phase composition in precursor of melten-iron catalyst for ammonia synthesis are investigated with high-pressure testing equipment, thermogravity, and XRD. It is found that the monophase of the iron oxide in precursor is an essential condition for the high activity and easy reduction of the iron catalyst for ammonia synthesis. The activity of the catalyst is consistent with its reduction performance, both of which have similar regularity change to phase composition ratios in the precursor. The easier the catalyst is reduced, the higher the activity. FeO-based catalyst with wustite phase structure has the highest activity and the easiest reduction of all the melten-iron catalysts for ammonia synthesis. The experimental results can be explained by phase composition ratio f, which was determined by the authors.