The invention discloses an image acquisition device and a focusing method. The image acquisition device comprises an image sensor, a lens module, a focusing driver, and a processing module, and is used to execute the focusing method. The image sensor is used to sense first spectral energy and second spectral energy related to an image at the same time, and is used to generate first image data and second image data respectively. The processing module is used to calculate a blur circle diameter difference value according to the first image data and the second image data, and is used to generate a control signal related to a target image distance according to an initial image distance, the above mentioned difference value, and a lookup table. The focusing driver is used to drive the lens module to move according to the control signal, and then the initial image distance is changed into the target image distance, and therefore the focusing is completed quickly, when a distance between a shot object and the lens module is short.