A reverse osmosis water purifying system includes a base with a support platform on which a water container is put and a purifying unit is received in the base so as to provide purified water into the water tank. A water level control device has a magnetic float unit connected to the water tank and the magnetic float unit has a magnetic float which can be inducted by a first magnetic-spring switch and a second magnetic-spring switch located on the base. The first magnetic-spring switch is located below the second magnetic-spring switch. The magnetic float moves along with the water level and activates the first magnetic-spring switch or the second magnetic-spring switch respectively to control the pump to provide the purified water into the water tank.