A priority function control apparatus for a construction machine and a control method thereof are disclosed, which enable an operator to set the order of preference or the extent of preference of an optional device in the case where a work is done through replacement of the optional device attached to an excavator. The priority function control apparatus includes a hydraulic pump and a pilot pump; an optional device and an attachment connected to the hydraulic pump; an attachment operation lever outputting an operation signal when an operator performs an operation; an optional device spool controlling a flow direction of hydraulic fluid that is supplied from the hydraulic pump to the optional device; an attachment spool controlling an amount and a flow direction of the hydraulic fluid supplied from the hydraulic pump to the attachment; an electrical operation device for the optional device; an electro proportional reducing valve outputting secondary signal pressure that corresponds to the operation signal of the electrical operation device for the optional device; a first pressure detection sensor detecting in real time signal pressure depending on the operation amount of the attachment operation lever to output a detection signal; and a controller limiting the signal pressure that is applied from the electro proportional reducing valve to the optional device spool by the operation of the electrical operation device for the optional device to a preset signal pressure if a priority function is applied depending on the operation amount of the attachment through detection of the first pressure detection sensor.