The introduction of a hose model to the Optical Virtual Private Network（OVPN）brings about a topic of optimization

i.e.minimizing the total bandwidth reserved for its users on the premise of ensuring the QoS.On the basis of the traditional Hierarchical Iterative Spanning Tree（HIST）algorithm

this paper proposes an approximate solution algorithm

named Bandwidth Constraint Spanning Tree（BCST）

which traverses all the spanning trees of the entire network physical topology by using the iterative method

and by adopting apruning scheme

finds the spanning trees with relatively smaller cost and time delay.Finally

it verifies the feasibility of the BCST algorithm by practical case simulation and comparison.