The invention discloses a seeding rate adjusting device for a seed metering device. The seeding rate adjusting device comprises a spindle, an outer perforated wheel, an inner perforated wheel, a rotating drive mechanism and a locking mechanism, and the outer perforated wheel, the inner perforated wheel, the rotating drive mechanism and the locking mechanism are rotatably mounted on the spindle. The outer periphery of the outer perforated wheel is provided with through holes, the outer periphery of the inner perforated wheel is provided with grooves corresponding to the through holes, and the inner perforated wheel sleeves the outer perforated wheel rotatably, slidably and internally. The rotating drive mechanism is fixedly connected with the inner perforated wheel, and an output end of the locking mechanism slidably presses the inner perforated wheel to realize fixed connection between the inner perforated wheel and the outer perforated wheel. The inner perforated wheel is capable of rotating relative to the outer perforated wheel to realize a staggered, intersected or overlapped positional relation between the grooves and the through holes, and accordingly size adjustment of seed holding spaces between the through holes and the grooves is realized. The seeding rate adjusting device has the advantages of electrical regulation control, convenience in operation, high seeding efficiency, safety and reliability.