A water-based ink for use in a computer-to-plate inkjet printing and a preparation method therefor. A polymer of 5 to 40 wt %, an additive of 0.01 to 10 wt %, a dye or pigment of 0.01 to 10 wt %, an organic solvent of 1 to 30 wt %, an anti-foaming agent of 0 to 5 wt %, and deionized water are stirred and mixed at room temperature. When the polymer is dissolved, the mixed solution is filtered. The filtrate is the water-based ink. The water-based ink is sprayed via a computer-to-plate machine onto a surface of a metal substrate (which may be an aluminum plate, a zinc plate, or a copper plate) to form a graphic region. A printing plate allowing for computer-to-plate printing is acquired after solidification. The printing plate acquired is allowed to achieve a halftone dot reproduction rate between 2 and 99% and a recognition rate of 175 lpi.