Determination of dimethoate in water was performed using liquid-liquid extraction coupled with gas chromatography(LLE-GC) and liquid-liquid extraction coupled with gas chromatography-mass spectrometry(LLE-GC-MS). Key parameters in the standard methods of Water quality—Determination of organic phosphorous pesticide in water—Gas chromatography(GB/T 13192—1991) and Water quality—Determination of 28 organophosphorus pesticides—Gas chromatography mass spectrometry(HJ 1189—2021), including injection mode, inlet maintenance, liner type, dehydration with anhydrous sodium sulfate, and nitrogen blow-down concentration, were systematically optimized. Experimental results demonstrated that stepwise rinsing of anhydrous sodium sulfate during liquid-liquid extraction improved the recovery of dimethoate from 60.4%~83.9% to 95.6%~108%. Thorough cleaning of the GC inlet with acetone significantly enhanced chromatographic response and improved instrument reproducibility. The use of glass wool liners with splitless injection mode markedly increased measurement accuracy. New liners required pretreatment with high-concentration standard solutions to deactivate surface active sites. After method optimization, the LLE-GC method achieved the detection limit of 0.09 μg/L with spiked recoveries of 94.3%~105% for real samples, while the LLE-GC-MS method showed the detection limit of 0.33 μg/L with recoveries of 89.4%~107%. Our optimized method meets the requirements of standard methods in terms of detection limit, precision, and accuracy, and is therefore applicable to dimethoate determination in surface water, wastewater, and seawater samples.