The system of steady state photorefractive spatial optical solitons can be descr ibed by a singular Lagrangian.To our knowledge,the commutation relations and qua ntum equations of motion are given by using corresponding principle, however it is not satisfactory since the constraints are ignored.In this paper,the commutat ion relations and quantum equations of motion are derived based on the Dirac the ory of constrained systems.The perturbation solution in screening soliton case i s found and the squeezing properties are discussed.