In this study, a functionalized ionic liquid (IL), 3-methyl-1-(ethoxycarbonylmethyl) imidazolium bis(trifluoromethylsulfonyl)-imide ([MimCH2COOCH3][NTf2]), was rationally designed and explored as an extraction solvent in dispersive liquid-liquid microextraction (DLLME) combined with ultra-high performance liquid chromatography (UHPLC) for the sensitive determination of organophosphorus pesticides (OPs). As π-π stacking interaction between the parent imidazolium cation core and the OPs is one of the most important factors, the proposed IL exhibited a high extraction efficiency. Moreover, during the DLLME process, a cloudy solution containing fine drops of [MimCH2COOCH3][NTf2] allowed for a larger contact area between the OPs and the IL, which accelerated the mass transfer, and therefore the enrichment could be realized in a rapid fashion. Under the optimal extraction conditions, the developed method was successfully applied in the analysis of OPs in environmental water samples with a high enrichment factor (more than 400), good recovery and reproducibility.