Small silica particles may be inhaled into the engine when helicopters and other aircraft are in the process of takeoff or landing on the gravel ground. The particles collide with engine blades at a relative speed more than 200 m/s, thus eroding the blades seriously. The particle velocity is hard to reach 200 m/s with straight-wall nozzle when total pressure is 0-6 MPa. So supersonic convergentdivergent nozzle is designed to increase the particle velocity. Particles motion in and out of the nozzle and throat optimization are studied by twophase simulations. The research shows that particles are speeded up mainly in the divergent part of the nozzle, and the acceleration of particles not only depends on the relative speed of gas and particles, but also the gas density. The throat radius is optimized, and the particles can be accelerated to 218 m/s by the nozzle with 0-001 6 m throat radius.