Potassium (K) deficiency damaged membrane stability through irregular reactive oxygen species (ROS) caused by K deficiency stress while osmotic adjustment and antioxidant capacities play an essential role in preventing plants from osmotic stress and oxidative damages. To investigate the difference of osmoprotectants and antioxidant enzyme activities in the root, two representative maize varieties, 90-21-3 (K-tolerant) and D937 (K-sensitive), were hydroponically cultivated under normal K (+K) and K deficiency (-K) treatments in Shenyang Agriculture University, China. The results showed that root accumulation, soluble protein in root of 90-21-3 and D937 were decreased under K deficiency stress, but the root to shoot ratio, proline, free amino acid, soluble sugar, reactive oxygen species (ROS) in root for both genotypes were increased. Compared with the root of D937, the root of 90-21-3 was able to swiftly accumulate more proline, free amino acid and soluble sugar in the root when encountering K deficiency. The antioxidant enzyme activity in the root of 90-21-3, including superoxide dismutase (SOD), and catalase (CAT), peroxidase (POD), were significantly increased to counter increased levels of O2·- and H2O2 under K deficiency stress. The presented results indicated that osmotic regulator and antioxidant enzyme were actively responded to K deficiency stress, 90-21-3 (K-tolerant maize) accumulated more osmoprotectants and enhanced the activity of antioxidant enzymes to degrade ROS, alleviating oxidative stress.