Abstract: Root shrinkage cavity in Al alloy electron beam welds is one special kind of welding defect, which results in stress concentration, induces the occurrence of crack and remarkablely decreases the carrying capacity, so it must be paid great attention. With the help of ANSYS APDL, by combining keyhole mode welding heat source model and interdendriticflowing theory as well as Niyama criterion, a set of defect criterion program based on temperature field was proposed to predict the root shrinkage cavity in 2219 Al alloy non-penetration electron beam overlaying weld. The results indicated that the simulated welding penetration has an error of 8.76%, the erros of simulated welding width is 13.28%, which werebasically in accordance with experimental samples. These results validated the exactness and feasibility of the defect criterion program developed in this paper.