Based on the principle of air refractive gradient measurement, a non-contact flame temperature field measurement instrument is designed using the projective background-oriented schlieren technique. A laser diode is used as the light source. Based on the CCD fast imaging and the particle image velocimetry technique, the offset of particles in the photograph is obtained to quantify the deflection angle. The refractive index gradient is obtained by Radon transform. The temperature field in each region of the flow field is obtained directly using the nonlinear curve fitting equation of air refractive index to temperature. The three-dimensional non-uniform temperature field is reconstructed by the back-projection algorithm. Thus, the visualization of flame temperature measurement is achieved.