The invention discloses a constant-flow drain valve which comprises a water valve cavity, a water valve water inlet connector, a filter screen, a water valve drain connector, a water valve liquid outlet, a valve
pipe, a valve column, a
plunger valve and a floating ball. The water valve water inlet connector is arranged at the upper end of the water valve cavity, the filter screen is arranged in the water valve water inlet connector, the water valve drain connector is arranged at the lower end of the water valve cavity, the water valve liquid outlet is formed in the water valve drain connector, the valve
pipe is arranged on the water valve liquid outlet, the valve column movably penetrates into the center of the valve
pipe, the
plunger valve is arranged at the top of the valve column, and the floating ball is arranged on the
plunger valve. The plunger valve is lifted directly through the
buoyancy of the floating ball, the
buoyancy of the floating ball and opening of the plunger valve are in direct proportion, opening of the plunger valve, the open area of the valve column and the valve pipe and the drain amount of the valve column and the valve pipe reach a
balance point, water can be drained automatically and stably, water outlet flow is constant, the design is simple, jamming is reduced, air leakage is avoided, observation and detection are facilitated, and the constant-flow drain valve can be used for experiment detection, and is simple in structure, convenient to main, low in maintenance cost, high in sealing performance, long in service life, and high in work efficiency.