Capacitors are playing increasing roles in our daily life. However, their high costs limit their wide application. Al is very cheap and the most abundant metal element on the earth. There is few report on Al-ion capacitor due to the challenges to find suitable anode with large capacitance and good rate performance. Here, the feasibility of assembling an Al-ion capacitor with good electrochemical performance is demonstrated. The Al-ion capacitor is enabled by using a composite of MoO3 nanotubes coated by a conductive polypyrrole (PPy@MoO3) as an anode, which functions via a redox intercalation/deintercalation of Al3+ ions in aqueous solution. It delivers a capacitance of 693 F g-1, about 3 times higher than that of electrode materials for sodium-ion capacitor in aqueous solution. Combined with activated carbon (AC) cathode, the Al-ion capacitor presents an energy density of 30 Wh kg-1 and an excellent cycling life with 93% capacitance retention after 1800 cycles. This finding could provide another energy storage device with low cost and promote the application of capacitors.