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By measuring and analyzing infrared thermal image of the specimen in static load tensile test process, it was studied that the in-fluence of the undercut defects and double-sided dressing method on the deformation behavior of the laser welded joint specimens of TC4 ti-tanium alloy. The results showed that for the unmodified specimens, the yield phenomenon occurs first in the region of the joint, but the un-dercut value has an effect on the stress and strain of starting to yield phenomenon, and a great effect on the plastic deformation behavior. When the undercut is less than a certain value, the large plastic deformation occurs in the base metal region and the plasticity of the speci-men is comparable to that of the base metal, but the larger undercut defect results in a concentrated plastic deformation in the joint region and rapidly failed in this region. But the double-sided dressing specimen is significantly different. The physical yield is no longer concentrated in the joint region, but at the same time occurs in the several regions including joint and the base metal. And the plastic deformation mainly oc-curs in the base material area, similar to that of the base material.