In the twoway relay network (TWRN), it brings the additional selfinterference when improving the spectrum effectiveness. The selfinterference cancellation for amplifyandforward TWRN is considered. In order to cancel the residual selfinterference with the imperfect channel estimation, a blind interference cancelation scheme for TWRN using orthogonal precoding is proposed. In the new scheme, the transmit data blocks at the source node are precoded in postmultiplying,with the target signal and selfinterference signal projected onto the different orthogonal subspace that is independent of channel state information (CSI). With unknown CSI, the target signal and selfinterference are decoupled, and the residual selfinterference is canceled to improve the system robustness in TWRN with the imperfect channel estimation. Moreover, the closeform expression and asymptotic expression of the average bit error rate (BER) are derived and the impact of imperfect channel estimation of different schemes is analysed. Numerical results validate the correctness of the derived expression.