Worms is a serious and growing threat to network and the traditional anti-virus technologies don't currently scale to deal with the worm threat. Friendly worm becomes a new active countermeasure. A diffusing balance-tree-based friendly worm propagation strategy is proposed, and the auto-generation rules and survivability enhancements are also given to account for the expansibility and the stability. As a case study, a simulation on worms cross-spreading is given. The results prove that the balance-tree-based propagation strategy is much more effective. Compared with the traditional ones, it accelerates the propagation speed and reduces the traffic impact on network.