Abstract: In order to investigate the effects of "ex-situ" toughened on the structural bearing capacities of composite stiffened panels, three point bending tests were conducted on composite omega stiffened panels without "ex-situ" toughen and with "ex-situ" toughen firstly, and the initial debonding load, critical failure load and the phenomenon of tests for the two kinds of stiffened panels were compared. Then, three-dimensional progressive failure model was established using ABAQUS, and the mechanisms of interface delamination and the failure process of the skins were analyzed considering the failure of stringer-skin interface and laminated composite plates. The results show that the finite element results coincides well with the test results, "ex-situ" toughen can only improve the initial debonding load of the interface slightly, but has relatively obvious improvement on critical failure load. At the beginning stage of delamination, the type I tensile mode plays a major role. With the delamination expanding continuously, the proportion of type II shear mode increases constantly, while the high toughness of shear mode is the significant cause for the outstanding performances of "ex-situ" toughened interface. The conclusions obtained show that "ex-situ" toughened interface has favorable delaminating resistance, and the behaviour on shear direction is particularly outstanding.