A process for producing an alkylene oxide by olefin epoxidation, wherein said process comprises the steps of: (1) in a first olefin epoxidation condition, in the presence of a first solid catalyst, a first mixed stream containing a solvent, an olefin and H2O2 is subjected to an epoxidation in one or more fixed bed reactors and/or one or more moving bed reactors until the conversion of H2O2 reaches 50%-95%, then, optionally, the resulting reaction mixture obtained in the step (1) is subjected to a separation to obtain a first stream free of H2O2 and a second stream containing the unreacted H2O2, and the olefin is introduced to the second stream to produce a second mixed stream, or optionally, the olefin is introduced to the reaction mixture obtained in the step (1) to produce a second mixed stream; (2) in a second olefin epoxidation condition, the reaction mixture obtained in the step (1) or the second mixed stream obtained in the step (1) and a second solid catalyst are introduced to one or more slurry bed reactors to conduct an epoxidation until the total conversion of H2O2 reaches 98% or more, with a proviso that said process for producing the alkylene oxide by olefin epoxidation has an selectivity for the alkylene oxide of 90% or more.The process of the present invention combines the slurry bed reactor with the fixed bed reactor and/or the moving bed reactor so as to overcome the disadvantages of the low conversion of H2O2 in the case that only the fixed bed reactor and/or the moving bed reactor are used, and the low selectivity for the target alkylene oxide in the case that only the slurry bed reactor is used.