An experiment has been performed to examine the phenomenon of the after-injection (secondary injection) of diesel fuel at the closing stage of needle motion in a single-spring diesel injector. A simplified analytical model has been proposed to describe needle bouncing. Needle bouncing was confirmed by spray visualization to be the primary cause of after-injection. The needle motion was well simulated by the proposed model. A series of parametric studies have been performed to find the effects of injector design parameters and operating conditions. Needle bouncing becomes small with a larger damping force by fluid shear at the needle surface. It can also be reduced by decreasing the material stiffness and installation displacement of the needle spring, the equivalent stiffness of the needle seat and the injector body and the maximum needle lift. The possibility of optimizing the injector design to reduce after-injection is discussed.