Alkali metal (Li, Na, and K) ion batteries are vital in portable and large-scale stationary energy storage. Recently, organic anodes have attracted increasing attention for alkali metal ion batteries due to their chemical diversity and potential high capacity. In this work, we discovered that cyclized polyacrylonitrile (cPAN) can serve as a superior anode for alkali metal ion batteries. Remarkably, upon activation cycling, as an anode of lithium-ion battery, cPAN exhibits a reversible capacity as high as 1238 mAh g-1 under a current density of 50 mA g-1. Based on electrochemical experiments and first-principles calculations, it is demonstrated that the hexagonal carbon ring, piperidine ring, and pyridine nitrogen in ladder cPAN are the main active sites for lithium-ion storage. In addition, we show that cPAN displays a unique potential-dependent solid electrolyte interphase formation from 0.1 to 0.01 V vs. Li/Li+. Furthermore, cPAN displays decent performance as an anode in SIBs and PIBs. The discovery of cPAN anode could pave the way for the future development of organic anodes for alkali metal ion batteries.