The electrochemical properties of α-graphdiyne (α-GDY) for potassium ion storage were studied by first-principles calculations. Initial potassium ion (K+) insertion was dominated by K-carbon (C) ionic interactions, which was gradually replaced by K–K metallic interactions with further insertion. During the K+ insertion process, a portion of sp-hybridized C was transformed to sp2-hybridized C, while the working voltage gradually decreased to <0.5 V. Two layers of K+ were adsorbed on both sides of an α-GDY monolayer with low diffusion barriers of <0.25 eV, which resulted in a high theoretical capacity of 2870 mA h ∙g−1 with the formation of C14K18. Trilayer α-GDY exhibited a smaller theoretical capacity of 1700 mA h g−1, based on the formation of C42K32 resulting from both interlayer and surface K+ storage. Large hexagonal C-rings provided a small K+ diffusion barrier of 0.353 eV. As a result, K+ ions quickly “dropped” into the interlayer space across the α-GDY layer, while K+ intercalation between α-GDY layers was not favored due to a large diffusion barrier of 0.666 eV, resulting from the small interlayer distance of trilayer α-GDY. This study demonstrated promising electrochemical properties of α-GDY for potassium ion batteries, particularly for its large specific capacity and high rate capability.