The determination of the combined effect of prometryn and oxadiazon was studied based on Sun Yun-pei’s methods. We took the pot-cultured Sagittaria trifolia which propagated by corm as target and screened thebest formula by the five-level two factors orthogonal rotation central composite design. The result showed that themixture of prometryn and oxadiazon got a synergistic effect on Sagittaria trifolia especially when the ratio was 1:0.9.The best range of dosage was: 361.05-440.325 g/hm² prometryn + 340.7-393.5 g/hm² oxadiazon. The best ratio ofthe mixture of prometryn and oxadiazon to control Sagittaria trifolia which propagated by corm was 440.325 g/hm² prometryn + 393.5 g/hm² oxadiazon, and the control effect was 82.33%. Mixture of the two herbicides never showedantagonism effect with any ratio.