The invention discloses a method for preparing glyphosate by PMIDA catalytic oxidation, comprising the following steps: 100-110 parts of PMIDA, 5-12 parts of activated carbon and 0.25-1.80 parts of pro-oxidants are added in 300 parts of water by mass part, the mixture is stirred, and then oxygen-containing gas with 70-100 percent of oxygen mass fraction is vented to the mixture to carry out catalytic oxidation reaction, the reaction temperature is 50-100 DEG C, the reaction pressure is 0.5-2.0MPa and the reaction time is 1-2h; after the reaction is finished, the solution is cooled and filtered to obtain filtrated mother liquid and a filter cake 1, the filtrated mother liquid is utilized in a circulating way, the filter cake 1 is hot-melted after water is added and then is filtered to obtain hot filtrate and a filter cake 2, the hot filtrate is cooled and crystallized to obtain solid glyphosate, and the filter cake 2 is recycled and used repeatedly. The method has stable product quality, can greatly improve utilization rate of oxygen and speed up the reaction rate; the production cost is low, formaldehyde can be effectively degraded, thereby meeting the industrial production requirements.