Potassium plays an important role in the process of protecting plant species from ammonium toxicity. In the present study, primary root, lateral root (LR) and shoot developments of Arabidopsis wild type (Col-0) were analyzed under excessive NH₄⁺ (30 mmol/L) with two concentration ratios of NH₄⁺/K⁺ (7.5 and 150) in agar medium. The results showed that under high ammonium condition (30 mmol/L), high NH₄⁺/K⁺ ratio (150) increased the ammonium stress, inhibited the shoot and root development and aggravated the oxidative stress compared with low NH₄⁺/K⁺ ratio (7.5). Compared with the low NH₄⁺/K⁺ ratio, the high NH₄⁺/K⁺ ratio inhibited the primary root elongation by 57.4%, the relative number of LRs by 33.3% and the shoot fresh weight by 69.9% under the high ammonium condition. The results of DAB (3, 3'-diaminobenzidine) staining of leaf indicated that compared with no ammonium treatment, the different external K⁺ levels (0.2 and 4.0 mmol/L) did not show significantly different effects on shoot oxidative stress of Arabidopsis; while under the high ammonium condition, the high NH₄⁺/K⁺ ratio further increased the H₂O₂ content of leaf and aggravated the oxidative stress compared with the low NH₄⁺/K⁺ ratio. Evans blue (EB) staining showed that the different external K⁺ levels did not show significantly different effects on shoot and root membrane permeability of Arabidopsis in the treatment without ammonium; while the high NH₄⁺/K⁺ ratio significantly enhanced shoot and root membrane permeability under the high ammonium condition. All the results indicated that high ammonium inhibited the development of shoot and root of Arabidopsis, and the inhibition would be enhanced by the high NH₄⁺/K⁺ ratio. In addition to the reduction in the absorption of ammonium by the plants due to presence of high concentration of potassium, the reason may be also ascribed to the exacerbated oxidative stress by the high NH₄⁺/K⁺ ratio. Therefore, appropriate NH₄⁺/K⁺ ratio plays an important role in response of plants to ammonium toxicity.