Relating to the technical field of corn field annual weed control, the invention provides a topramezone, nicosulfuron and atrazine compounded dispersible oil-based suspension agent and a preparation method thereof. The compounded dispersible oil-based suspension agent is prepared from 1.5 parts of topramezone, 4.5 parts of nicosulfuron, 22 parts of atrazine, 3 parts of fatty alcohol polyoxyethylene ether, 5 parts of styrylphenol polyoxyethylene ether, 0.5 part of xanthan gum, 0.3 part of sodium benzoate, 0.5 part of a defoaming agent and 62.7 parts of deionized water. The preparation method includes: pumping the raw materials into a preparation kettle to conduct mixing, then performing primary grinding by a colloid mill, and conducting fine grinding by a sand mill, carrying out sampling analysis, and when the sample is qualified, conducting filtering, metering, packaging and warehousing. According to the invention, the topramezone, nicosulfuron and atrazine compounded dispersible oil-based suspension agent can achieve an obvious synergistic action range when topramezone, nicosulfuron and atrazine are compounded in a ratio of 1.5:4.5:22 to a concentration of 28%, while preparations formed by mixing in other ratios cannot achieve the obvious synergistic action range.