In the tree network of IEEE802.11s using HWMP (hybrid wireless mesh protocol)as its routing protocol，PREQs (path requests)are simply flooded for maintaining the routes between leaf nodes，which causes PREQs broadcast storm problem. This paper proposes a TTL limited scheme to solve the problem. The scheme designs a method to grade the leaf nodes. With the help of table-driven routing in HWMP，the leaf nodes are divided into different grades according to their hop counts to the root node and the grade information is synchronized in the network. The source node no longer uses the default TTL in HWMP when sending PREQ，instead，it sets the TTL of the PREQ to the sum of its own grade and the destination node’s grade. Theoretical analysis and experimental results show that the scheme limits the delivery distance of PREQs and outperforms HWMP at overhead，loop delay，package loss rate and throughput in static tree network if the sum of the source node’s grade and the destination node’s grade is not large.