In a small electron storage ring, such as the Hefei Light Source (HLS) ring, the newly generated ions, which can not escape from the beam potential and then are trapped from tu to tu, will lead to the beam instability. The ions created by the leading bunches can perturb the trailing bunches and also themselves during their subsequentpassage, which will make the amplitude of beam oscillation be damped and anti-damped periodically. A computer simulation based on the strong-weak model shows a good agreement with our analytical model using the linear theory.