A hydraulic system of construction equipment having a float function is disclosed, which includes a hydraulic pump and a pilot pump supplying hydraulic fluid to drive a working device; a remote control valve outputting a pilot signal corresponding to an operation amount of an operation lever; a working device hydraulic cylinder connected to the hydraulic pump; a main control valve installed in a discharge flow path of the hydraulic pump to control a start, stop, and direction change of the hydraulic cylinder when a spool is shifted; a solenoid valve shifted to an opening position when a float function switch is turned on; and a two-stage port relief valve installed in a flow path between the main control valve and the working device hydraulic cylinder, and shifted to return the hydraulic fluid in a large chamber of the hydraulic cylinder to a hydraulic tank so that the working device descends when the float function switch is operated to be turned on.
