With the rapid development of distributed generators， the impact on power systems is not to be overlooked. The oscillation conditions of on-load tap changers（OLTC） are analyzed. The impact of distributed generators on aVt ， an important parameter defined as the sensitivity of bus voltage with respect to tap changer， is discussed. The result shows that the access of the distributed generators may have an effect on tap changer oscillation. As a result， a model of penetration level calculation considering tap changer oscillations is proposed. Finally， the 112-bus IEEE RTS is used to testify the effectiveness of the model proposed.