The determination approach for trinexapac–ethyl, and its degradation product, trinexapac was set up through the additive recovery experiment with HPLC–UV technology to understand their residual level and degradation rule in farmland soil. The residues of trinexapac–ethyl and trinexapac in farmland soil were firstly extracted using acidic acetonitrile, and the extract was purified with florisil dispersive solid phase extraction, then we employed HPLC–UV to detect their residue. The results showed that when the additive concentration of trinexapac–ethyl varied from 0.05 to 5.00 mg/kg, the additive recoveries changed from 79.68% to 103.55%, with the coefficient of variation varied from 1.26% to 10.22%; while, the trinexapac,s recoveries varied from 79.18% to 104.13%, and its variation coefficient varied from 3.29% to 10.75%. The approach met the technical needs for pesticide residue analysis and determination. A case study of the residue and degradation on trinexapac–ethyl and trinexapac in two kinds of typical farmland soil, i.e., quaternary red soil and alluvial soil from Hunan were conducted under indoor simulation culture experiment at 20 ℃ and dark condition, the results showed that their residue and degradation accorded with the first–order kinetic equation which was written as Ct=C0?e–kt, and the degradation half–life of trinexapac–ethyl was 8.92 days in quaternary red soil, and 7.67 days in alluvial soil; while those of trinexapac was 9.39 days and 8.59 days, respectively. Both the degradation rate of trinexapac–ethyl and trinexapac were faster in the two kinds of typical farmland soil, and it was no risk to the farmland soil environment.