Polylactic acid (PLA)/starch blends were prepared by melting blending using twin-rotor continuous mixing extruder, with starch and PLA as raw materials and glycerin as plasticizer. The microstructure, rheological properties, crystallinity and mechanical properties of the blends under different rotor speeds and glycerol contents were studied. The effects of shear force and plasticizer content on the two-phase compatibility of the blends were obtained. SEM, FT-IR and dynamic frequency scanning results of the blends were analyzed. The results show that the appropriate increase of rotor speed and glycerol content could improve the plasticization degree of starch and compatibility between starch and PLA. DSC curves show that the crystallinity of the blends decreases with the increase of glycerol content. When the glycerol mass fraction is 20% and rotation speed is 300 r/min, the compatibility between starch dispersed phase and PLA matrix is the best. The blend has good mechanical properties, with the tensile strength of 25.1 MPa and bending strength of 33.1 MPa.