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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/Mg4:0, K/Mg4:1, and K/Mg8:1, respectively). Compared with K/Mg4:1 treatment, the leaf chlorophyll content, net photosynthetic rate, and total biomass of tomato seedlings under K/Mg4:0 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/Mg4:1 treatment, total biomass of tomato seedlings of K/Mg8:1 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.