The invention discloses a method and a device for determining target regions in an image. A problem of a difficulty in precisely capturing a projection image when an electronic device is used for shooting in the prior art is solved. The method comprises steps of dividing an acquired image into multiple units and calculating an average brightness value of each unit; according to a corresponding relation between a preset average brightness value and a brightness grade, determining a brightness grade corresponding to the average brightness value of each unit; when the brightness grade is larger than or equal to a set first condition which number of units with preset threshold values satisfies in the image, determining at least one continuous brightness region form the image; selecting a continuous brightness region including most pixel points from the at least one continuous brightness region; and when the number of the pixel points included in the selected continuous brightness region satisfies a set second condition, using the selected continuous brightness region as a target region. Thus, the electronic device can be used for quickly and precisely capturing highlight images.