The flame propagation characteristic of polyethylene dust cloud inside a semi-open tube is studied in virtue of high-speed photographing technology. The rules of the influence of initial distribution range of polyethylene dust cloud on flame speed and shape is focused on. Studies show that flame propagation range generally exceed the initial distribution range of dust cloud and is positively correlated to it. Flames within certain dust cloud distribution range can accelerate continuously; however, flames will decelerate after its initial acceleration when dust clouds are too centralized or decentralized. When dust mass is constant, the maximum flame speed and average flame speed generally increase and then decrease with the increasing initial distribution range. Initial distribution range affects flame shape evolution significantly, such as its influence on the flame width along the radial direction, flame brightness, front smoothness, fault, etc. The conclusions provide scientific basis for flame monitoring and safety control at the initial stage of accident.