The invention discloses a method for determining pesticide residues on grains. The method comprises the following steps: step a, a grain sample is obtained through ultrasonic extraction; step b, 2-4 g of sample fragments are added to a centrifuge tube containing hydroquinone and sodium thiosulfate, then an acetic acid acetonitrile solution is added, and the mixture is oscillated vigorously and subjected to ultrasonic extraction and centrifugation; step c, an acetonitrile extracting solution obtained after centrifugation is fed into a centrifuge tube containing 1-3 g of a cleaning agent, the centrifuge tube is placed on an oscillator, oscillated for 3 min and left to stand for 3 min, an upper solution is absorbed and transferred into a syringe filter directly, suspended solids in the solution are removed through filtering, and a filtrate is collected in a test tube; step d, the test tube filled with the filtrate is placed in a water bath at the temperature of 38 DEG C. The sample is extracted with an ultrasonic technology, so that a large amount of flesh is prevented effectively from being brought in during determination, and the determining accuracy is high.