Abstract: This paper deduced the transient thermal balance equation for soldered joint under the constant laser pulse excitation, as well as analysis calculation method for defect size and thermal resistance based on the inverse problem of limit temperature difference. Then thermal analysis simulation of constant laser pulse excitation was conducted by establishing digital model for void-type defects. The evolution law among infrared temperature difference, thermal resistance and defect size of soldered joint was obtained. By preparing samples with pseudo soldering or defects, thermal test was conducted under different temperatures, and infrared temperature testing was carried out to analyze the evolution of soldered joint defects after thermal test. The results indicate that the evolution mechanism of soldered joint defect followed the change law similar to that of metal material creep curve. And the size of defects changed in the order of increasing, then decreasing, and then increasing again. The results could help to improve the thermal test theories for soldered joint.