n-Type organic mixed ionic-electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop. As a result, the figure-of-merit (µC*) of n-type organic electrochemical transistors (OECTs) lags far behind the p-type analogs, restraining the development of OECT-based low-power complementary circuits and biosensors. Here, two n-type donor-acceptor (D-A) polymers based on fused bithiophene imide dimer f-BTI2 as the acceptor unit and thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. The cyanation of TVT enables polymer f-BTI2g-TVTCN with simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property. As a result, it is able to obtain a high volumetric capacitance (C*) of 170 ± 22 F cm<sup>-3</sup> and a record OECT electron mobility (μ<sub>e,OECT</sub> ) of 0.24 cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup> for f-BTI2g-TVTCN, subsequently achieving a state-of-the-art µC* of 41.3 F cm<sup>-1</sup> V<sup>-1</sup> s<sup>-1</sup> and geometry-normalized transconductance (g<sub>m,norm</sub> ) of 12.8 S cm<sup>-1</sup> in n-type accumulation-mode OECTs. In contrast, only a moderate µC* of 1.50 F cm<sup>-1</sup> V<sup>-1</sup> s<sup>-1</sup> is measured for the non-cyanated polymer f-BTI2g-TVT. These remarkable results demonstrate the great power of cyano functionalization of polymer semiconductors in developing n-type OMIECs with substantial electron mobility in aqueous environment for high-performance n-type OECTs.