In the fast time-varying channel, there exists interference among subchannels of the orthogonal frequency-division multiplexing (OFDM) system. The problem of inter-subchannel interference (ICI) also appears in the single-carrier (SC) block transmission with frequency-domain (FD) equalization. However, not all ICI reduction algorithms for the OFDM system can be directly applied in the SC block transmission. In this paper, we first consider a previous ICI self-reduction algorithm for the OFDM system. Though this algorithm can be applied in the SC block transmission, it extends the duration of each time-domain (TD) signal to add the diversity of FD symbols for ICI self-reduction. However, unlike the OFDM system, the SC block transmission can only use the block type pilot arrangements, and extending the original TD duration of signal may increase the relative time variance, and worsen the CR estimation the follows the ICI reduction. Based on the characteristics of SC block transmission, we propose an efficient ICI self-reduction algorithm. The proposed algorithm is also able to add the diversity of FD symbols to make the ICI self-reduced. Without extending the original duration of the TD signal, the algorithm does not affect the performance in the subsequent CR estimation. Similar to , when the variation of each path is linear with time within the block interval, we can show that the ICI can be completely removed. Our ICI self-reduced algorithm gives almost no additional complexity comparing with the typical SC block transmission with FD equalization. The simulation results show that the proposed algorithm attains better performance than the original ICI self-reduction algorithm that extends the signal duration in the high-mobility channel.