This cordless grass trimmer is designed for outdoor use, trimming grass on large lot, such as parks. Powered by an environment-friendly lithium battery, each trimmer is also equipped with a disc motor, eliminating the need for a power-transmitting shaft and achieving lower noise, lower vibration, higher efficiency and lighter weight than existing products as well as greatly reducing operator fatigue. The telescopic rod, easily contractible to 560 mm, facilitates carrying and storage. Combining with the high capacity backpack power supply enables longer runtime than existing grass trimmers.