The high ammonia nitrogen wastewater in the landfill leachate RO concentrate contains high potassium and sodium ions, but the effect of potassium and sodium ions on the crystallization of struvite remains unknown. In this paper, the addition of magnesium salts and phosphates to the simulation of different potassium and sodium concentrations and the actual RO concentrate promotes the crystallization of struvite. The concentrations of ammonia nitrogen, phosphate, potassium and sodium ions in the solution before and after crystallization were determined. The structure and morphology of the crystallized products were analyzed by XRD and SEM. The effects of potassium and sodium ions on the crystallization and ammonia recovery of struvite were discussed. The results showed that, with the increase of potassium ion concentration, the ammonia nitrogen removal rate decreased from 96.5% to 83.0%, and the mass percentage of potassium ions in the precipitated product gradually increased from 2.4% to 6.4%. The characteristic peak of struvite is shifted to a high angle, indicating that potassium ions enter the lattice position of struvite and form a K-struvite; due to the competition between potassium ions and ammonium ions, the removal rate of ammonia nitrogen is reduced under the same magnesium, ammonia nitrogen and phosphorus ratio conditions. However, potassium ions have little effect on the crystal size and morphology of struvite. Sodium ion has no significant effect on ammonia nitrogen recovery, but it will affect the crystal morphology of struvite. Experiments with the actual landfill leachate RO concentrate show that the competition of potassium ions is considered in the water treatment of the recovery of ammonia nitrogen from struvite crystals, and the ratio of magnesium salt, ammonia nitrogen and phosphate is increased by 1.8:1:1.5. The recovery rate of ammonia nitrogen is ensured and, at the same time, the potassium ion is partially recovered as a nutrient component of the slow-release fertilizer.