Abstract: In order to reduce the weight of aircraft structures effectively, the skeleton of aircraft is treated in the conceptual designof strengthening frame. By analyzing the concept of`gradient threshold' and pointing out its weakness, the author proposes theconcept of `modified gradient threshold' and combines it with the strategyof `constraint compensation' to form a new kind of topology optimizationalgorithm for structural layout of a strengthening frame. The topologyoptimization results of several examples show that their layouts aregood. We can see that, using the combination of CAD/CAE and topologyoptimization for aircraft strengthening frame conceptual design is feasibleand that the proposed `modified gradient threshold' topologyoptimization algorithm is effective.