The classical state in tensor product space of considering quantum systems and ancillary quantum systems was constructed.By taking partial trace over ancillary composite quantum systems
the problem of detecting entanglement for states in infinite dimensional bipartite composite quantum system was studied.A necessary and sufficient condition for separable states in infinite dimensional bipartite quantum systems was obtained.The above work extended the relevant results got by Li Nan and Luo Shunlong
and reinforced the exploration of entanglement for states in infinite dimensional quantum systems.