A hydraulic system for construction equipment having a float function is provided, which can perform ground leveling as making a boom descend due to its own weight without using hydraulic fluid that is discharged from a hydraulic pump. The hydraulic system for construction equipment having a float function includes first and second hydraulic pumps (1,2) and a pilot pump (3), an operation lever (8), a float function switch which selects a float function, a main control valve (4) installed in a discharge flow path of the first and second hydraulic pumps (1,2) and including a boom spool (4-1) controlling a start, step, and direction change of the boom cylinders (5,5a), a solenoid valve (7) shifted when the float function switch is turned on, a float valve (9) installed in a flow path between the boom spool (4-1) and the boom cylinders (5,5a) and shifted by pilot signal pressure to return the hydraulic fluid to a hydraulic tank (24), and a holding logic poppet (20) mounted in the float valve (9) to prevent an abr
