Potassium (K) and magnesium (Mg) levels and their balances are two factors affecting the growth of plant.  However, the responses of different crop cultivars to K/Mg ratios are less clear.  This study was aimed at assessing the different responses of tomato (Solanum Lycopersicum L.) cultivars to the different K/Mg supply ratios.  Three tomato cultivars (Zhongza 9 (ZZ), Gailiangmaofen (MF), and Jinpengchaoguan (JP)) were grown in pots with three different K⁺/Mg²⁺ ratios (4:0, 4:1 and 8:1, represented by K/Mg_4:0, K/Mg_4:1, and K/Mg_8:1, respectively).  Compared with K/Mg_4:1 treatment, the leaf chlorophyll content, net photosynthetic rate, and total biomass of tomato seedlings under K/Mg_4:0 treatments were decreased by 69.7, 89.1, and 53.1%, respectively.  The Mg deficiency symptoms were observed when the Mg content in shoot became lower than 4 mg g^–1 DW.  Compared with K/Mg_4:1 treatment, total biomass of tomato seedlings of K/Mg_8:1 treatment was decreased by 21.6%; the shoot and root Mg contents were decreased by 10.4 and 21.8%, respectively; and Mg uptake of tomato was reduced by 34.1%.  There were significant differences in biomass and Mg uptake for the three cultivars between the different K⁺/Mg²⁺ treatments.  The Mg uptake of the three different cultivars ranked as ZZ>JP>MF under Mg deficiency and high K condition.  In conclusion, the growth and Mg uptake and allocation of tomato were influenced significantly by imbalance K and Mg supply. JP and ZZ were the cultivars with the highest efficiency in Mg uptake.