Based on the mechanical and physical properties study of forage grass seeds, a multi-line with one-device type metering device was designed. It was composed of adjustable screw, stirrer, metering device housing and central metering sheave and so on. The sowing rate can be set by turning the screw to change working length of central metering sheave relative to metering device housing. The stirrer inside of the sheave housing was used to prevent seeds overhead. And metering of different sizes of seed was adjusted by changing the position of internal components of slot wheel mechanism. Innovative design on structure of the central metering sheave was finished. According to the structure parameters and physical characteristic parameters, different seed sowing rate of per hectares was calculated. And then the working length scale of the central metering groove wheel was made. And there was a one-to-one correspondence between scale values and sowing quantity per hectare of different kinds of seed.