<p>Gas cylinders are typical pressure vessels used for transporting and storing gases (including liquefied gases) and serve as essential infrastructure supporting a country’s economic and social development. Repeated filling and discharging in use may cause material fatigue, leading to plastic deformation and eventual failure. Permanent volumetric expansion is a key safety and quality indicator for cylinders. Traditional methods, whether direct expansion method or water jacket method, require dedicated equipment and sites, cannot realize in-situ inspection, and cannot capture localized plastic deformation. They are also inefficient and costly. To enable accurate in-situ measurement of permanent volumetric expansion, this paper proposes a method based on 3D point cloud modeling, combining laser scanning with theoretical analysis and experimental comparison. Using 3D laser scanning, point cloud data of five cylinders were collected before and after hydraulic testing. Triangular mesh models were constructed from the data, and permanent volumetric expansion was derived from volume change calculations. Experimental results compared with the water jacket method show that the proposed method uses fewer devices, is simple to operate, and yields small errors, confirming its feasibility. More importantly, unlike existing methods that only reflect overall deformation, this method accurately and intuitively reveals deformation at each local section of the cylinder. This is significant for precisely assessing cylinder safety. The method demonstrates clear advantages over similar techniques and offers engineering practicality. Future work may establish corresponding measurement and evaluation standards based on this method’s technical features.</p>