The present invention provides an overflow control method for frame synchronization of base stations in wireless communication network system. The wireless communication network system includes a radio network controller, a first base station, a second base station, and a mobile unit. The radio network controller transmits a plurality of data frames to the first base station and the second base station. The present invention includes the following steps. First, determine whether number of the plurality of data frames received by the second base station is larger than a judging storage capacity. Second, calculate an X and delete X of data frames from the second base station if the number is larger. The X is equal to the result of subtracting the judging storage capacity from the number. Meanwhile, detect a link quality between the mobile unit and the first base station. If the link quality is lower than a preset value, calculate an N and delete N of data frames from the second base station when N is larger than zero. The N is equal to number of data frames not yet deleted from the second base station and already received by the mobile unit.