The invention relates to an iron-molybdenum catalyst precursor, and an iron-molybdenum catalyst and applications thereof. The precursor comprises the metal complex of Fe, Mo and Cr, and Cr2O3, whereinan atomic ratio of Mo/(Fe+Cr) is 1.51-1.9, and the particle size distribution (calculated as volume fraction) comprises that the particles with a particle size of less than 20 [mu]m account for lessthan 20%, the particles with a particle size of 20-60 [mu]m account for 60-85%, and the particles with a particle size of more than 60 [mu]m account for less than 25%. According to the present invention, the precursor is prepared by using an ammonium molybdate solution, a Cr<3+> solution and ferric nitrate-containing saturated gas as raw materials through a specific reactor, wherein the raw materials are beaten into fine liquid droplets and gas bubbles through the blade wheel of the reactor, such that the prepared catalyst precursor has the uniform and centralized particle size distribution soas to easily improve the comprehensive performance of the catalyst; and with the application of the catalyst prepared by roasting the precursor in the formaldehyde preparation through oxidation of methanol, the high activity, the high selectivity and the high stability can be provided.