Using flat sweeping frequency ultrasonic technology, degradation of dimethoate in aqueous solution under ultrasonic field at different ultrasonic frequencies, fixed/sweeping modes, radiate solution volumes, radiate time were investigated. Radical mechanism under ultrasonic field was studied. The results showed that degradation efficiency at sweeping frequency ultrasound mode was proved to be superior to that at fixed mode, and the efficiency of double ultrasonic plats was better than the simple plat. The better treatment system of ultrasonic degradation operating parameters were frequency 68kHz, power 80W (corresponding to the processing volume of 8L), sweeping period 100ms, frequency combination of 68kHz with 68kHz, radiate time 180min. Under these conditions, the degradation rate of dimethoate with initial concentration 3mg/L reached 71%. Mechanism study results indicated that the degradation of dimethoate in aqueous solution by ultrasonic was partly caused by free radical oxidation.