To test the differences in root exudation under Cd stress among four lettuce varieties, a hydroponic culture experiment was conducted. In this study, four lettuce varieties were cultivated in Hoagland’s solution containing 10 mg·L^-1 Cd for 3 weeks. The water culture method was used to obtain a solution containing root exudates at the end of every week. In all four cultivars, the roots’ secretion capacity increased over time, but the diversity of compounds in the root exudates and the soil-Cd activation capacity of root exudates decreased. The roots’ secretion capacity was significantly negatively correlated with the soil-Cd activation capacity of the exudates (r = 0.78, P<0.01) and with the number of compounds in root exudates (r = 0.59, P<0.01). The soil-Cd activation capacity of root exudates was significantly positively correlated with the number of compounds in root exudates (r = 0.69, P<0.01). Thus, under Cd stress, the lettuce root secretion capacity increased while the diversity of compounds in root exudates decreased, resulting in decreased soil-Cd activation.