The flame spread above oil surface is affected by the ambient wind, including the velocity and direction of the wind. In current paper, the experiments of flame spread above jet fuel of RP-3 with different pool widths are conducted to find out the variations of flame tilt angle under the influence of longitudinally forced air flows. It is found that as the longitudinally forced air flow speed increases from 0 to 2.57 m/s, the flame tilt angle increases from 0 degrees to about 75 degrees, regardless of directions of air flows. The new non-dimensional parameter, characteristic size ratio lambda = d/D is introduced into the Welker et al.'s model. The tangent value of the flame tilt angle is directly proportional to alpha(Ri)(-beta)(d/D)(gamma) under both opposed and concurrent flame spreads. The mechanism of flame inclination under the effect of longitudinally forced air flow is analyzed according to the coupling of combustion and gas flows. The flame inclination is principally determined by the coupled effects of vertical buoyancy and horizontally forced air flow.