How to determine the future open boundary conditions of the diffusive equation for certain bays or estuaries is still a question, although the present conditions can be easily obtained by observation.In this paper, a new concept “the effective reach” led to the conclusion on theopen boundary for the case n- 1 was confirmed by some numerical experiments. This rational boundary condition for simulating the pollutants diffusive fields can be effectively applied to environmental problems. How to determine the future open boundary conditions of the diffusive equation for certain bays or estuaries is still a question, although the present conditions can be easily obtained by observation. This paper, a new concept “the effective reach” led to the conclusion on theopen boundary for the case n- 1 was confirmed by numerical study. This rational boundary condition for simulating the pollutants diffusive fields can be effectively applied to environmental problems.