Abstract: Due to an intensive increase of crop yields, an application of nitrogen and phosphorus fertilizer is gradually growing, resulting in imbalance of fertilizer application. It urgently needs to adjust the proportion of nitrogen and phosphorus fertilizer application and call for the combined application of potassium-magnesium fertilizer. Magnesium ions are found in the center of chlorophyll molecules, and as chlorophyll is an essential component in the reaction of photosynthesis, which produces energy for growth, magnesium ions are therefore essential. Potassium is needed for chemical reactions of the plant cell and helping to transport other ions needed by the plant. It helps in the formation and movement of carbohydrates. Potassium and magnesium facilitate plant energy production by improving the production and regeneration of ATP, the biochemical battery molecule that stores and releases energy supply of plant. Focusing on the combined application of fertilizer in farmland for increase of crop yields and fertilizer use efficiency, we conducted a field experiment to study an effectiveness of combined application for potassium-magnesium fertilizer on different crops in Xi’an and analyzed an optimal application amount for different crops. The results showed that the combined application of potassium-magnesium fertilizer has significant effect on wheat, corn and cucumber. Crop yields of wheat increased by 13.54%, 22.56 % and 9.81% after application of 375kg ha-1, 750kg ha-1and 1125 kg ha-1 potassium-magnesium fertilizer, crop yields of corn increased by 5.9%, 11.56% and 16.09% after application of 750 kg ha-1 , 1125 kg ha-1and 1500 kg ha-1 potassium-magnesium fertilizer. Application of same quantity fertilizer from the different potassium-magnesium fertilizer varieties has significantly increased the yield of cucumber based on the same amount of N, P. Application of potassium-magnesium to cucumber has an significant increasing yield by 29.06 %. The yield of cucumber increased by 9.7％, 6.3％ and 18.85％ by application of only potassium sulfate of 437 kg ha-1, magnesium sulfate of 360 kg ha-1 and the combined application of potassium sulfate of 437 kg ha-1 with magnesium sulfate of 360 kg ha-1. The treatment with fertilizer of magnesium potassium sulfate provides higher yield and quality than that only with potassium sulfate. The yield of Kiwifruit increased by 13.81％ after application with potassium-magnesium of 2.0 kg per plant. The relative fruit quality an