Via fertilizing different potassium levels, this paper studied the action pathway of sugar accumulation and the suitable fertilization time. This study was used Gala fruit as test materials, and applied different levels of potassium fertilizer. We tested fruit weight, soluble solids, titratable acid, sugar metabolites and their related enzyme activities. The research showed that potassium element should be fertilized before 84 days after bloom, and the higher concentration of potassium, the more significant promotional effects on fruit quality. What is more, potassium could influence fructose contents via SDH activity, SS activity and AI activity. The study not only provided theoretical basis for potassium to regulate fruit growth and development, but also provided theoretical guidance for apple yield and quality improvement.