A cooperative relaying system based on non-orthogonal multiple access (NOMA) is considered a promising technique in next-generation wireless communications. In this letter, we investigate the performance of a dual-hop decode-and-forward based on the NOMA relaying system in terms of achievable rate and power allocation coefficient over imperfect estimation of κ- μ shadowed fading channels. Moreover, we consider imperfect successive interference cancellation at the relay in the first phase of transmission. Specifically, we derive the closed expressions for the average achievable rate and power allocation coefficient in terms of hypergeometric function. Finally, the numerical results confirm that our derived analytical results fit well the Monte Carlo simulations.