In view of the problem that the seal protection for a laser emission system with large-diameter and wide-band is very difficult, the purification positive pressure ventilation seal method has been put forward. This method was to achieve the purpose of sealing by exhausting the original air and making the pipe full of purified and dry and clean air, so the air and particles from outside will be resisted based on the positive pressure principle. For the flow field of the positive pressure ventilation in laser emission pipe, the numerical simulation of the continuous phase and discrete phase has been made by the CFD software-Fluent, found that the purified positive pressure airflow formed by the positive pressure ventilation could resist the air backflow from outside, but also made a barrier function for the dust particle with kinds of sizes in the air, when the concentration was less than 4.71E^-3 kg/m³. In addition, this paper found that the positive pressure ventilation flow caused a 0.32 μm optical path difference when laser′s band was within 0.8 -10.6 μm,based on the analysis of refractive index in the pipe. Measurement results from the positive pressure ventilation experiments show that trends of vary of particle concentration and simulation curve are basically consistent, which demonstrates the effectiveness of the method of positive pressure ventilation.