Poly(aryl-co-aryl piperidinium) (c-PAP) has recently attracted wide attention in anion exchange membrane (AEM) fuel cells (AEMFCs). Here, we report poly(aryl-co-terphenyl piperidinium)-x (PDnTP-x) AEMs with different alkyl spacers (n = 0, 1, 2, 6, 10) in the backbone. Long alkyl chains (n = 6 or 10) improved the dimensional stability and gas tightness (H2 permeability <10 Barrer) of membranes, while short-chain-type PDnTP-x membranes displayed higher ion conductivity (>150 mS cm−1 at 80 °C). All the PDnTP-x membranes possessed excellent alkaline stability (1 M NaOH at 80 °C for 1000 h) and oxidative stability (200 h in Fenton's reagent). Short-chain-type PD0TP-x based AEMFC reached a new-record power density of 2.67 W cm−2 at 80 °C in polyaromatic AEMFCs, which is higher than that of long-chain-type cells (1.7 W cm−2). The short-chain-type PDnTP-x AEMFCs can run stably under a 0.4 A cm−2 current density for 220 h with a low voltage decay rate of ∼77 μV h−1. Display omitted •Elucidated the effect of alkyl chain in polymer backbone on fuel cell performance.•PDnTP-x showed excellent stability (1000 h in 1 M NaOH, 200 h in Fenton reagent).•PD1TP-25-based AEMFC reached a new-record power density of 2.67 W cm−2 at 80 °C.•PD2TP-25-based AEMFC can run for 220 h with a voltage decay rate of 77 μV h−1.