A portable electronic device for measuring a distance of an object by performing an auto focus function and a distance measuring method are provided. The portable electronic device includes an image pickup module and a distance measuring program. The image pickup module includes a lens, an optical sensing element and a transmission mechanism. The distance measuring program includes a distance look-up table. The distance look-up table is established by correlating a plurality of lens moving step numbers with corresponding object distance values. The auto focus function is performed to capture an image of an object by the image pickup module, thereby obtaining a current lens moving step number required for performing the auto focus function. A current object distance value corresponding to the current lens moving step number is acquired according to the distance look-up table.