Abstract: In order to study the effect of topdressing potassium chloride on the yield and quality of lettuce in different growth periods and provide scientific basis for the production of high yield and good quality of leaf vegetables, field experiments were set up to study effects of topdressing potassium chloride fertilizer on yield，quality and nutrient of lettuce in different growth periods. Results indicated that compared to basal application (CK2) treatment, yield of lettuce were reduced topdressing potassium fertilizers in different growth periods. Yield of lettuce were gradually reduced along with the change in lettuce growing periods applying potassium fertilizer, and yield reduction of lettuce were gradually increased with the period of applying potassium fertilizer shift after. Effect on vitamin C and amino acid content of lettuce leaf with different periods applying potassium fertilizer were different. The treatment of largest soluble sugar content of lettuce was basal application (CK2). But nitrate content of lettuce were reduced with topdressing potassium chloride fertilizer in different periods, chloride could inhibit the formation of NO3-_N and uptake of root. Applying potassium fertilizers in different periods on total nitrogen content of lettuce leaf had little change, the treatment of highest total nitrogen content of lettuce stem was basal application (CK2). And the treatment of highest total phosphorus and potassium content of lettuce was basal application (CK2). So potassium chloride as basal application improved nitrogen, phosphorus, potassium content. Total chloride content of lettuce was improved along with the period of applying potassium fertilizer shift after.