Abstract: Aiming at problem that opening of water plug-in gate valve in coal preparation plant needs manual adjustment and cannot achieve centralized control, a line laser assisted visual monitoring method for opening of water plug-in gate valve in coal preparation plant was proposed. Line laser is used as an auxiliary detection means to increase target detection characteristics and avoid influence of light changes on target detection, and video monitoring system is used to collect plug-in gate valve image. Template matching method is used to locate target in the image, adaptive threshold method is used to detect line laser projected on the gate, and then adjacent disconnected areas of the line laser on the gate are connected through morphological operations such as corrosion, expansion and so on. Meanwhile, Gaussian filtering method is adopted to remove image noise, so position and length of the line laser in the image are obtained to detect opening of plug-in gate valve. PLC centralized control system realizes closed-loop control of opening of plug-in gate valve by comparing the detected opening value and the set one. The experimental results show that the method can accurately locate plug-in gate valve under different background brightness conditions, and improve detection accuracy of opening of plug-in gate valve.