Based on the photothermal displacement effect and the optical diffraction effect, an alteative pulsed photothermal technique has been developed. This technique overcomes some disadvantages of conventional methods and provides a sensitive, noncontact and nondestructive probe of the transient thermal and mechanical process at solid and thin film surfaces. Meanwhile, a three-dimensional theoretical model has also been presented to describe the experimental phenomena. Excellent agreement between the theory and the experiment is obtained. As an example, the technique is demonstrated by measuring the thermal diffusivity of aluminum and brass thin plates.