Potassium (K) is an essential nutrient and abundant cation in plant cells. The application of K+ could alleviate abiotic stress. However, it was reported that the alleviation of K+ on salt-stressed plants only happened when K+ concentration was low. Most studies were focused on effects of sodium salts on plants in salty soils, and little information was reported about potassium salts, especially a higher level of potassium in alkaline salts. To explore the effects of K+ in alkaline salts on plant growth, and whether it had a same destructive impact as Na+, we mixed two alkaline sodium salts (ASS) (NaHCO3:Na2CO3 = 9:1) and two alkaline potassium salts (APS) (KHCO3:K2CO3 = 9:1) to treat 10-day-old wheat seedlings. Effects of ASS and APS on growth, photosynthesis, ions absorption and solutes accumulation were compared. Results indicated that effects of potassium salts in soil on plants growth were related to K+ concentration. Both growth and photosynthesis of wheat seedlings decreased, and the reduction was higher in APS treatment than in ASS treatment at 40 mM alkalinity. ASS treatment absorbed Na+, competing with K+ and free Ca2+, and inhibited the absorption of inorganic anions. APS treatments accumulated K+ and reduced the absorption of anions, with no competition with other cations. Both APS and ASS treatments promoted free Mg2+ accumulation and inhibited H2PO4−uptake. The reduction of H2PO4− promoted organic acid synthesis indirectly. Soluble sugar and proline accumulation were also related to the alkaline condition and extra K+ addition. In conclusion, excess potassium ions in soil, especially in alkaline soils, were harmful to plants. APS was another severe salt stress, intensity of which was higher than ASS. The growth and physiological response mechanisms of wheat seedlings to APS were similar to ASS. Both inorganic ions and organic solutes took part in the osmotic adjustment. Differences for APS depended on K+, but ASS on Na+.