Helianthus tuberosus is well-known Asteraceae plant for its high environmental adaptation ability. It is easy to cultivate, which tempts gardeners to simply leave them completely alone to grow. However, growth and development of this plant is also sensitive to cadmium. In order to evaluate the regulation effects of brassinosteroid on plant cadmium resistance of H. tuberosus, this experiment studied the regulation effect of exogenous foliar spraying 24-brassinosteroid (24-EBL) on seedling dry weight, root shoot ratio (R/S), photosynthetic pigment content, leaf gas exchange parameters, water use efficiency (WUE), cadmium (Cd) content of two H. tuberosus varieties (NY2 and NY5) under 100 μmol/L cadmium (Cd) stress. The results showed that: (1) Compared with control, the plant dry weight, R/S ratio, photosynthetic pigment content, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and WUE were decreased, while the values of intercellular CO₂ concentration (Ci) were increased in NY2 and NY5 seedlings under Cd stress. (2) Compared to only Cd treatment, the plant dry weight and R/S ratio were increased to varying degrees with exogenous spraying different 24-EBL concentrations (10^-10, 10^-9, 10^-8, 10^-7 mol/L) under Cd stress. 10^-9 mol/L 24-EBL (EBL2) and 10^-8 mol/L 24-EBL (EBL3) treatments made the dry weight of NY2 and NY5 peaked, and 50% and 64% higher than the values of only Cd stress treatment, respectively. (3) Compared to only Cd treatment, 24-EBL application increased the contents of both chlorophyll (Chl) and carotenoid (Car) in two H. tuberosus seedlings under Cd stress, and also increased their Pn, Gs, Tr in NY2. However, 24-EBL decreased the values of Ci in both H. tuberosus seedlings, and Ci decrease of NY5 showed more significantly than that of NY2. (4) Under Cd stress, 24-EBL increased the WUE values of NY2 and NY5 in varing degrees, respectively, and WUE increase of NY5 showed greater than that of NY2. (5) Under Cd stress, Cd accumulation in new expanded leaves of NY5 was much higher than that of NY2, and EBL2 reduced its Cd content of NY2, but EBL3 enhanced its Cd content of NY5. (6) Under Cd stress, foliar spraying 24-EBL significantly decreased Cd content in all of organs of NY2 and in root of NY5, but did not markedly affect Cd content in the other organs of NY5. Generally, the suitable concentration of 24-EBL could significantly improve cadmium resistance of H. tuberosus, and it was mainly due to improvement of photosynthesis and water use caused by 24-EBL. Furthermore, compared with NY2, the better amelioration of non-stomatal restrictions on the NY5 plant applied with 24-EBL leads to its better improvement of photosynthesis and WUE, and its higher growth stimulation under cadmium stress. The results that NY5 plants has much larger biomass, more Cd accumulation and better biomass increase applied with 24-EBL under cadmium stress than NY2 also implied that NY5 can be considered as an excellent candidate for phytoremediation of metal-contaminated soils.