It is often reported that a jet fire occurs in industrial installations or in the transportation of hazardous materials and could amplify the scale of accident by imposing lots of heat on people and nearby facilities. This paper presents a new semi-empirical radiation model, namely, the line source model to predict the radiant heat flux distribution around a vertical turbulent hydrocarbon jet flame. In terms of the fact that the jet flame holds the large ratio of flame length to diameter, the new model assumes that all thermal energy is emitted by a line source located inside the jet flame volume. With three typically different shapes to simulate the jet flame shape, a formula is proposed to characterize the profile of the emissive power per line length (EPPLL), by which the line source model can be closed in theory. In comparison with the point source model, the multipoint source model, and the solid flame model, the new model agrees better with the measurement of the heat flux radiated from a small jet flame. It is found that the line source model can well predict the radiant heat flux of both small and large jet flames, yet with the flame shape simulated by the back-to-back cone and the cone–cylinder combined shape, respectively. By parameter sensitivity and uncertainty analysis, the ranking by importance of input parameters is also given for the new model.