A method of magnifying the deflective angle of a light beam is proposed, in which the measurement sensitivity of speckle photography can be raised. By speckle Photography, a 3-D asymmetrric temperature field for the phase object is measured. And the temperature distribution for a giving layer (z=const) is obtained by convolution backprojection algorithm. The experiment shows that this method is suitable for the measurement of the phase object with the small sectional size and the small temperature gradient.