Abstract: A device for controlling the discharging amount of the flush water from toilet tank includes a U-shaped member secured to a lateral wall in the tank and close on two lateral sides by a pair of U-shaped plates, a large rotor, an eccentric wheel and a small rotor disposed into the U-shaped member, a small float ball connected to the eccentric wheel, a trip lever connected to the small rotor, a check plate pivoted to one end of the U-shaped member, a large flush hand connected to a cylindrical body of the large rotor and a small flush handle connected to a positioning rod of the small rotor to be actuated by the large flush handle. When press the large flush handle, a large amount of flush water can be discharged and when press the small flush handle, a small amount of flush water is discharged.