Trichoderma harzianum T-soybean plays an important role in controlling soybean root rot disease. However, the mechanism by which it improves plant tolerance to salt stress is not clear. In this study, we investigated the possible mechanism of T-soybean in mitigating the damage caused by salt stress in Cucumis sativus L plants. Our results suggest that T-soybean improved salt tolerance of cucumber seedlings by affecting the antioxidant enzymes including peroxidase (POD) (EC 1.11.1.6), polyphenol oxidase (PPO) (EC 1.14.18.1), phenylalanine ammonia-lyase (PAL) (EC 4.3.1.5), catalase (CAT) (EC 1.11.1.6), superoxide dismutase (SOD) (EC 1.15.1.1), ascorbate peroxidase (APX) (EC 1.11.1.11), and glutathione reductase (GR) (EC 1.6.4.2), by increasing the levels of proline, soluble sugars, soluble protein, ascorbic acid (AsA) and chlorophyll as well as improving root activity. Treatment with T-soybean improved the ratio of glutathione (GSH)/oxidized glutathione (GSSG) and AsA/dehydroascorbate (DHA), and up-regulated the expression of CsAPX and CsGR genes involved in the AsA-GSH cycle. In addition, treatment with T-soybean increased the K content and K/Na ratio while decreased the Na concentration and ethylene level. In summary, the improved salt tolerance of cucumber plants may be due to multiple mechanisms of T-soybean, such as the increase in reactive oxygen species (ROS) scavenging, as well as maintaining osmotic balance and metabolic homeostasis under salt stress.