The basic structure and work principle of engine-driven constant frequency rectangular pickup baler were introduced. The general structure, pickup, feeding mechanism and pressing mechanism were designed and computed. The result indicated that the baler which could pick up, press and tie grass and crops straw had the characteristics of reasonable structure and high degree of automation. The fully bale rate of the baler was 99.1%，the regular bale rate was 98%，and bale density was 130～230kg/m3.