In order to explore the changing mechanism of rice allelopathic potential under potassium deficiency, allelopathic rice PI312777 and non-allelopathic rice Lemont were employed as donor plants and the morphological and physiobiochemical characteristics of receiver plant branyardgrass(Echinochloa crus galli) mediated by the two donor plants were investigated under low potassium stress (5 mg·L-1 K, denoted as K-) and normal potassium (40 mg·L-1 K, denoted as K+). The result showed that, under K- condition, the two rice accessions inhibited the growth of barnyardgrass to different extents. The inhibitory effect (IR, %) of allelopathic rice PI312777 on barnyardgrass was greatly higher than that of non-allelopathic rice Lemont, showing 56.19% (IR, the same below) VS 14.67% in the suppression of root length of barnyardgrass, 35.78% VS 9.27% in plant height, and 46.24% VS 9.47% in dry weight. Under K- condition, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and the root activity of barnyardgrass were descended at a higher rate when the target plants were co-cultured with allelopathic rice PI312777 than those with non-allelopathic rice Lemont. Furthermore, analysis of Real time Fluorescent Quantitative PCR(FQ-PCR) showed that, under K- condition, phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, hydroxylase, and O-methyltransferases, which are the key enzymes in phenolic metabolic pathway, were all up-regulated in root and leaves of allelopathic rice PI312777. But all these enzymes were down-regulated in non-allelopathic rice Lemont except phenylalanine ammonia-lyase in leave. HMG-CoA reductase, squalene synthase, monoterpenes cyclase, sesquiterpene cyclase, and diterpene cyclase, which are the key enzymes in isoprenoid metabolic pathway, performed the same expression pattern in roots of the two rice accessions, and it was also found that in leaves of non-allelopathic rice Lemont, HMG-CoA reductase was up-regulated and the other four enzyme genes were down-regulated. But in allelopathic rice PI312777, only squalene synthase showed down-regulated, the reverse was true in the other enzyme genes. The result from the analysis of phenolic acids in two rice root exudates by using HPLC showed that, the total amount of ten phenolic acids concerned in allelopathic rice PI312777 under K- condition, was 2.30 times as those as under the K+ condition. However it was only 0.91 times in non-allelopathic rice Lemont under the same condition. It is therefore suggested that enhancement of rice allelopathic potential in the suppression of the target weeds under K deficiency might be attributed to the up-regulation of the key enzymes involved in phenolic metabolism, which leaded to the activation of phenolic metabolism, and increased phenolic allelochemicals, consequently inhibited growth of barnyardgrass.