The Moire deflectometry has been widely used in the structural display and key parameter measurement of various flow fields, and there have been different researches for the placement of phase objects in the optical path. The theoretical and experimental studies were carried out on this issue. Theoretical analysis showed that when the measured object was placed in front of two gratings, the offset and phase mainly depended on the distance between two gratings, while the deflection angle was determined by the ratio of phase disturbance and the distance of the two gratings. However, when the measured object was placed between two gratings, the offset and phase mainly depended on the distance from the measured object to the next grating, and the deflection angle was determined by the ratio of phase disturbance and the distance from the measured object to the next grating. In order to verify the correctness and rationality of the theoretical results, a candle flame was taken as an example for experiments. Furthermore, the comparison shows that in two cases, the position of the measured object will not affect the final phase and deflection angle reconstructed in the experiment, as long as the distances mentioned above are suitably chosen. The relevant research results can provide a certain reference for the better use of Moire deflectometry in the detection of various flow fields.