Abstract: Different rice varieties (lines) with and without ALS mutation were selected as materials to select rice varieties (lines) with different resistance to imazamox and the corresponding working concentration. Then three amino acids (valine, leucine and isoleucine) were mixed as protective agent. On this basis, Bihu and brassinolide, which has a broad spectrum effect on plant growth and resistance, were mixed as protective agent Bihu. The degree of damage caused by imazamox to rice and the repair effect of protective agent were compared. Finally, the effects of protective agent on the growth, antioxidant enzyme, detoxification enzyme and acetolactate synthase (ALS) enzyme activities in different rice materials treated with imazamox were determined, in order to reveal the mechanism of the branched chain amino acid safety agent in alleviating the imazamox damage of different rice varieties. The results showed that sensitive varieties such as Nanjing 9108 are tolerant to a concentration of no more than 28.8 g a.i./hm² of imazamox, and resistant rice ones such as Jinjing 811, 73119 and K37 with ALS mutation sites are resistant to a dose of 144.0 g a.i./hm² of imazamox. According to the fresh weight of the treated plants, the three components and ratios of branched chain amino acid safety agent all showed significant alleviating effects on the harm of imazamox in rice, among which the best alleviating effect was achieved when leucine: isoleucine: valine=3∶1∶1, that is, when combined with 1125 g/hm² of branched chain amino acids, 675 g/hm² of L-leucine∶225 g/hm² of L-isoleucine∶225 g/hm² of L-valine=3∶1∶1, which can significantly alleviate the inhibition of growth of K37 by 100.0 g a. i./hm² of imazamox. Compared with the commonly used safety agents such as Bihu or brassinolide, the branched chain amino acid safety agent had a better effect. Furthermore, compared to the 4-leaf stage, the detoxification effect of spraying on the 5-leaf stage of rice was better, and it did not affect the effectiveness of killing the weedy rice. The main physiological mechanism of the branched chain amino acid safety agent in this study in alleviating the damage caused by imazamox in rice was not that it increased the target ALS enzyme activity, but that it not only induced an increase in the activity of key endogenous detoxifying enzymes, but also in antioxidant enzyme activity of rice. In Conclusions, by mixing or applying the branched chain amino acid safety agent using it during the 5-leaf stage of rice, it can alleviate the damage of imazamox to different types of rice, restore the affected plants to normal growth levels or close to normal growth levels, and do not affect the killing of the weedy rice.