A novel 3 D temperature field reconstruction method is proposed, which is based on multi wavelength thermometry and optical computed tomography. A mathematical model of multi wavelength thermometry is founded, and a novel optical computed tomography based on changeable relaxation parameter technique is proposed. Through computer simulation and comparison with algebraic reconstruction technique and filter back projection algorithm, the reconstruction results of the new method are discussed in detail. The study shows that the new method always gives the best reconstruction results. As an example, temperature distribution of a profile of four peaks candle flame is reconstructed with multi wavelength radiation computed tomography.