Solidliquid separating is an effective way to pretreat dairy manure. In order to further reduce the moisture content of separated solid and improve the efficiency of solidliquid separation, a new screw extrusion separator was designed based on the theory of changing diameter screw compression and material characteristics of cow manure. The screw extrusion separator was constituted by feed hopper, feed sleeve, separation sleeve, separation screen, screw axis, discharge device with pressadjusting spring, threephase motor and so on. Both the diameter of axis and the pitch of screw blade were changed in a linear fashion. The design equations of diameter and blade pitch were elaborated. The paper studied the relationship between separation performance and structure of screw axis, structure of separation screen, rotational speed of screw axis, press of discharge end. The separation performance was expressed in two main following aspects: separation efficiency and solid removal rate. An orthogonal experiment was done, which took taper of screw axis, clearance size of separation screen, rotational speed of screw axis and press of discharge end as factors. The experiment results indicated that the optimal condition for the separator was as follows: the taper of screw axis was 2.68°，the clearance size of separation screen was 1mm, the rotational speed of screw axis was 60r/min, and the press of discharge end was 3250N. Validation test showed that the separation efficiency was 746.18kg/h, the solid removal rate was 49.84% and the moisture content of separated solid was 61%. The results showed that the separator met the expected requirements and achieved running well. Compared with original separator, DN140 screw ertrusion solidliquid separators was more efficient, its solid removal rate was lower and power was higher. It would be an promising approach for pretreatment of cow manure by using the new machine.