According to the problem that processing moiré fringes based on phase reconstruction methods in moiré deflection tomography, the formation of moiré fringes is analyzed from the scalar diffraction theory. The zero-and first-order frequency spectrum is extracted respectively by pinhole filtering. Compared with results from the traditional geometric superposition principle of moiré effect, Result in this paper fits well and has a more accurate form. Results show that the moiré fringes are generated by shearing interference of multiple wave fronts and moiré patterns with first-order filtering are the strict cosinusoidal intensity distribution. Finally, the combustion of propane in the air is reconstructed by using the Fourier transform method which based on multi-grid phase unwrapping algorithm and the filtered back-projection algorithm of moiré tomography.