In this paper, we study the long time well-posedness for the nonlinear Prandtl boundary layer equation on the half plane. The initial data is the perturbation of a monotonic shear profile, we prove the existence, uniqueness and stability of solutions in weighted Sobolev space, the life span of the solution can be arbitrarily long while the initial perturbation of the monotonic shear profile is enough small. We use the energy method to prove the existence of solution by a parabolic regularizing approximation. The nonlinear cancellation properties of the Prandtl equations under the monotonic assumption is the main ingredients to establish a new energy estimate.