Abstract: Compared with the conventional one-way relaying system, cognitive two-way relaying networks can improve spectrum utilization further by applying network coding. This paper presents an analytical study on the sum rate for cognitive decode-and-forward (DF) two-way relaying networks with underlay spectrum sharing. The bidirectional protocol, i.e., three time-slot time division broadcast (TDBC), is considered. We proposed an optimization power allocation scheme, which maximizes the sum rate of secondary users (SUs) to afford the best communication performance under maximum tolerable interference power (MTIP) constraint to primary user (PU) and a total transmit power constraint to SUs. The optimal solutions to power allocation are obtained in different cases. Numerical simulations validate the theoretical analysis and demonstrate the total available power of SUs and the MTIP constraint have a great effect on the sum rate.