A method of automatically adjusting a depth of field, suitable for adjusting settings relevant to the depth of field when pre-shooting an image to be shot through a digital camera, includes capturing an image to be shot; dividing the image into a plurality of detected blocks; performing a statistic process for an object distance on each detected block, and recording an object distance value of each detected block; generating an object-distance cumulative curve according to the object distance value of each detected block; performing a smoothing process to eliminate peak curves generated in the object-distance cumulative curve; looking up a mode of all the object distance values and a corresponding detected block according to the object-distance cumulative curve; adjusting corresponding aperture and focal length of the digital camera according to the corresponding detected block. The user shoots images at different sites without adjusting the digital camera into corresponding shoot mode.