A method of preparing a catalyst for producing acrolein by oxidation of propylene at high space velocity, said catalyst is a Mo—Bi—Fe—Co based composite metal oxide. Producing unsaturated aldehyde via partial oxidation of lower unsaturated olefin at high space velocity using said catalyst is suitable for process with or without off-gas recirculating. Said catalyst is prepared by co-precipitation, the reaction conditions for using said catalyst to produce unsaturated aldehyde are, the space velocity of unsaturated lower olefin relative to catalyst being 120˜200 h-1(STP), reaction temperature being 300˜420° C. and absolute pressure being 0.1˜0.5 MPa; a single-stage unsaturated lower olefin conversion ratio of greater than 98.0% and carbon oxide yield of less than 3.3% with an overall yield of unsaturated lower aldehyde and acid of greater than 94.0% are obtained. The process to prepare the said catalyst is simple, easy to be repeated, and capable of industrial scale-up.