The production and transportation processes of gas fuel may be faced with a safety challenge in the form of a jet fire from pipeline leakage. In actual accidents, sometimes some obstacles may appear in the path of the horizontal jet fire development leading to impingement. The spread mechanism of such a flame is complex due to the coupling relation of the wall resistance force, buoyancy force and initial momentum. Experiments were conducted to study a horizontal jet flame impinging a wall as a function of fuel initial velocity and nozzle-wall spacing. Results show the transient development of flame morphology and spread mechanism of the steady flame. The upward buoyancy force of the horizontal jet flame gradually increases with the fuel as it moves further away until flame impinges the wall. From the side view, flame morphologies include the flame impinging the wall (up -down spread and up spread stages) and flame not impinging the wall (free spread stage). A counterclockwise flame vortex occurs due to the competition between the upward buoyancy force and downward momentum below the nozzle projection point. From the front view, the flame morphology changes from Umorphology to Omorphology. By combining the knowledge of the jet fluid, momentum conservation principle and Newton's second law, a flame size model of the horizontal jet flame impinging the wall is proposed. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.