Stochasticity of magnetic field lines near the separatrix in the Toroidal Divertor Tokakamak is investigated by the island overlappng model and the separatrix mapping method. Both results indicate that the undisturbed confinement core region can exists, and the width of stochastic region is found to be controllable by the x-point position and ploidal magnetic field. The width of stochastic layer obtained by the more relaible separatrix mapping method was larger than that determined by the island overlapping model.