In order to improve the practicability using the optical computer tomo graphy (OCT) to diagnose thermal flow fields,non-linear auto-adjusting iterative reco nstruction technique (NAIRT) is proposed and applied to reconstr uct the temperature distribution on a cross section of a thermal flow field.The field is detected with a collimated laser beam.After passing through the field,the collimated la ser beam disturbed by the field is diffracted by two identical equi -spaced Ronchi gratings.The diffracted patterns were projected onto a screen,so we can get d eflected Moiré fringes.To obtain the phase shift data,the Moiré patterns are analyzed with a series of mathema tical transforms,including Fourier transformation,filtering,inverse Fourier transf ormation and reconstruction with the positive first order spectra,and so on.The phase shi ft data are transformed into deflected angles,namely,the optical projections.The refractive index distribution is reconstructed by NAIRT with the deflected angles,then is transformed into an air density distribution,and finally transformed into the temperature distribution. As a result, NAIRT could effectively reconstruct the temperature distribution on a cross sect ion of the field.With the same processing methods,NAIRT could effectively reconstruct arbit ary cross-sectional temperature distribution of the field.Therefore,NAIRT could ef fectively diagnose the field.