In this letter, we investigate a reconfigurable intelligent surface (RIS) assisted downlink multi-user multiple-input single-output (MISO) system. We derive the approximations of average rate of user equipments (UEs) under proper assumptions, and reveal the relations between average rates and system parameters. Furthermore, a RIS configuration algorithm is proposed to improve the average sum rate with low complexity by exploiting the approximation of average rate and statistical channel state information (CSI). The rationality of the assumptions, the tightness of the approximations and the effectiveness of the proposed algorithm are validated by numerical results.