To prevent dust and moisture to enter the laser emission pipe of a high-energy laser emission system with large-diameter and wide-band, a positive pressure ventilation seal method for the laser emission pipe is proposed. This method uses purified, dry and clean air to exhaust original air, and allow a positive pressure flow on an emission port to resist the air diffusion into the laser emission pipe from outside and to achieve the sealing for the pipe. The numerical simulation and analysis for a flow field of the positive pressure ventilation in the laser emission pipe are performed by the CFD software-Fluent, and it shows that the clean air keeps a certain velocity and pressure on the emission port and could form a positive pressure airflow to resist the backflow from outside. For a long wavelength 10.6 μm laser, the variation of refractive index in the pipe is estimated to be a magnitude of 10-7 based on the pressure change, which causes a 1 μm optical path difference. The simulation results are experimentally verified and the results show that the velocity varies of interior have nearly no difference, though its' velocity is a litter bigger than that of simulation, and there is a positive pressure airflow with 0.64 m/s.Moreover, the relative humidity of clean air has dropped to 29% from 59%, which basically reaches the design sealing requirements of the positive pressure ventilation system for dust-proof and dehumidification.