The invention relates to an interstage separation device suitable for small-diameter aircrafts. The device is jointly composed of a connecting and unlocking device and an impulse separating device, and can be widely applied to the small-diameter aircrafts such as multi-stage rocket projectiles and multi-stage target projectiles. When an aircraft flies, connection and separation of a boosting stage and an upper stage are achieved through the interstage separation device. The connecting and unlocking device mainly comprises an upper-stage connecting cabin, a lower-stage connecting cabin, an unlocking top disc, steel balls, explosive bolts, ball pads and locking nuts; and the impulse separating device is mainly a cylindrical spring. When the aircraft flies in a whole rocket mode, the upper-stage connecting cabin is connected with the upper stage in a fastened mode, and the lower-stage connecting cabin is connected with the boosting stage in a fastened mode. The unlocking top disc radially supports the six steel balls in an upper-stage connecting locking hole and a V-shaped groove of the lower-stage connecting cabin, so that inter-stage axial limiting is realized; and a shear pin on the upper-stage connecting cabin is inserted into a radial limiting groove of the lower-stage connecting cabin, and interstage radial limiting is achieved. After the aircraft completes boosting flight at the boosting stage, the explosive bolts are detonated and separated, the unlocking top disc moves forwards under the action of the cylindrical spring, the steel ball radial support is relieved, the steel balls fall into an unlocking top disc groove, and the aircraft completes stage unlocking; and the cylindrical spring pushes the unlocking top disc to drive the upper-stage connecting cabin to continuously move until the upper-stage connecting cabin and the lower-stage connecting cabin are separated, and the aircraft achieves reliable interstage separation.