The effects of biochar addition on the physicochemical properties of coastal saline soil and the activity of the antioxidation system in maize seedlings were studied by pot experiment, and the typical coastal saline soil sampled from Yellow River Delta was taken as test soil. A total of six treatments were set up in the experiment, including non-fertilizing (CK), fertilizing without biochar (CFK) and fertilizing with different addition rates of biochar (the addition rate was 12.5, 25.0, 50.0 and 100 g/kg, respectively). The results showed that:(1) Compared with CK, with the increasing of biochar-addition rate, the electrical conductivity (EC), available potassium, available phosphorus, cation exchange capacity (CEC) and easily oxidized organic carbon (EOC) of coastal saline soil increased significantly. The pH and alkali hydrolysable nitrogen content of soil treated with biochar showed decreased trends compared with CK, but there was no significant different between the biochar treatments. However, biochar addition had no significant effect on exchangeable sodium compared with CK.(2) Moderate addition rate of biochar (12.5, 25.0 and 50.0 g/kg) increased the activities of antioxidant enzymes (SOD, POD and CAT) and root activity in maize seedlings, and reduced the production rate of superoxide anion and hydrogen peroxide content of leaves, thereby improving the physiological characteristics of maize seedlings. However, the higher addition rate of biochar (100 g/kg) exhibited a negative impact on the activity of antioxidant enzyme in maize seedlings, resulting in the higher accumulation of reactive oxygen in the plants. Therefore, moderate addition rate of biochar could improve the physical and chemical properties of coastal saline soil, and to a certain extent, effectively improve the physiological characteristics of maize seedlings under salt stress, but the higher addition rate could inhibit the antioxidant system of crops.