LH-2010A is a newly developed fungicide with novel mode of action in the treatment of cucurbit downy mildew. Dissipation kinetics and residue levels of LH-2010A in cucumber were investigated using a QuEChERS method with GC-ECD. Field trials were constructed at three different sites in China in 2013 and 2014. The average recoveries of LH-2010A in fortified samples were between 94.0 and 106.2% for cucumber and between 84.4% and 98.7% for soil, with relative standard deviations within 3%. The dissipation rate of LH-2010A residue was evaluated assuming a pseudo first-order kinetics. The half-lives of LH-2010A were 2.8–4.2 days and 6.3–9.4 days in cucumber and soil samples, respectively. The terminal residues in cucumber were 0.077–0.207 mg/kg and 0.109–0.307 mg/kg on the first day after spray at the recommended dosage and 1.5 times of the recommended dosage, respectively. Using this rapid and sensitive method, we determined the dissipation kinetics and residue level of HL-2010A in cucumber. The suggested MRL value of HL-2010A in cucumber is 0.5 mg/kg. The research would provide guidance for proper and safe use of this newly developed fungicide in cucumber in greenhouse ecosystems.