Considering the problem of limited spectrum and relay resources, we proposed a scheme using incremental relay protocol to improve the secondary network outage performance. Firstly, a CR-NOMA network system model combined with incremental relay protocol is constructed. The primary network and the secondary network share the same relay for communication. When the user of the secondary network fails to receive information, the node closest to the shared relay is selected as an incremental relay to assist transmission. Secondly, the exact expressions of outage probability and throughput are derived, and the Monte Carlo method is used to verify the results. Based on the outage probability and throughput of the secondary network, the transmission power ratio between primary and secondary networks and the power distribution of shared relay are optimized. Compared with the average power allocation schema, it is proved that the optimized schema has advantages in improving the outage performance and throughput. Through simulation and analysis of outage performance with or without using incremental relay protocol, it is concluded that adding an incremental relay in the shared relay network can reduce the probability of system outage and improve network reliability.