Buoyancy causes a horizontal jet flame to bend upward when it loses sufficient initial momentum. Therefore, the variation in flame trajectory must be considered to accurately predict the thermal radiant flux of a horizontal jet flame governed by buoyancy and momentum. We introduce the flame trajectory equation into the linear source equation of radiation to establish a new model. The flame length ratio is calculated by flame trajectory length, horizontal projected flame length, and vertical projected flame height in the new model. This paper also presents a formula for flame lift-off distance and flame radiant fraction. We further list the geometric parameters of the horizontal-kite shape, tile-kite shape, and flame trajectory models. Comparing the experimental data and model predictions shows that the flame trajectory model is more accurate than the horizontal-kite and tile-kite models, especially when the Froude number is low. We also study the effects of flame lift-off distance and flame length ratio on predictive accuracy. The flame trajectory model computes quickly, and a normal laptop computer needs only 1.08 s to get a picture with 24,000 data points.