When wind speed, soil particle size distribution, collection time and inlet collection efficiency etc. are certain, separation efficiency of sand sampler is the main factor affecting collection efficiency, which is often affected by wind-sand separation rules. In order to reveal wind-sand separation rules in the sand sampler, a sand separator in the shunt-hedging and multi-stage expansion combined sand sampler was took as a study model, a analysis of single-phase flow field and gas-solid two-phase flow field in the sand separator was made through Fluent numerical simulation and micro wind-tunnel test. On the base of explaining reducing velocity mechanism of airflow, a further analysis of airflow reducing velocity and sand volume fraction effect on wind-sand separation was done, and the influencing situation of different particle size of sand was expounded. The results showed that reducing velocity substantially was the most effective method that achieving wind-sand separation, the main reason caused by substantial airflow speed reduction was the formation of a large value of turbulent kinetic energy field;for wind-sand flow, when the sand content was high, the wind-sand separation effect was going to be good with good velocity reducing effect, when sand was affected by strong winds, sand with particle size less than 0.03241mm was easily induced by flow field, less affected by the motion of inertial, the possibility excreted from the exhaust port was large, which was the main part that affected sand separation efficiency and sand collection efficiency. The study provided a theoretical reference for the design of sand sampler, which had a good collection performance, and it further improved the theoretical system of sand sampler technology.