Worst-Case User (WCU) at the intersection of cells usually experiences a low signal quality and then low performance. Improving the performance of the WCU is important problem for the network operator to secure the Quality of Service. This paper proposes a RIS-assisted cellular network model, where the RIS is located at the position of the second nearest Base Station of the WCU, to improve the performance of this user. Thus, the coverage probability of the WCU is derived as the function of the density and properties of obstacles, number of RIS elements and the reflection ratio of each RIS element. The analysis results show that the proposed system can significantly improve the user performance without increasing power consumption.