A micro-spherical iron-based catalyst and a preparation method thereof are disclosed. The catalyst contains a potassium promoter, and at least one transitional metal promoter M which is one or more kinds of metals selected from Cr, Cu, Mn and Zn. It also contains a structure promoter S, which is SiO2 and/or Al2O3, wherein both of SiO2 and Al2O3 are modified by MoO3, TiO2 and/or ZrO2. The weight ratio of components is Fe:M:K:S=100:3-50:1-8:3-50, in which the metal components are calculated based on metal elements, the structure promoter is calculated based on oxides. The catalyst is prepared by co-precipitation method.