As model drug，beclomethasone dipropionate(BDP)nanoparticles were prepared by using the antisolvent precipitation method in a T-junction microchannel. The influence of surfactant on particle morphology，as well as the influences of surfactant concentration，BDP solution flow rate，antisolvent flow rate，BDP solution concentration and precipitation temperature on particle size were explored. The results indicated that the morphology of BDP was spherical with the addition of surfactant(HPMC).Besides，the particle size decreased with decreasing BDP solution flow rate，increasing antisolvent flow rate and decreasing precipitation temperature. However，with the increase of BDP concentration，particle size reached a minimum. BDP nanoparticles with an average size of 200—260 nm and narrow size distribution could be prepared under the following conditions: BDP solution flow rate of 4 ml·min-1，