The grass sliding cutting angles of oblique rotary blades are analyzed and it is pointed out that, the static grass sliding cutting angle of oblique rotary blade is the same as that of normal rotary blade, the dynamic grass sliding cutting angle of lengthwise edge of oblique rotary blade is the angle between velocity vector and normal plane of the edge curve, and there is not a definite decrement Δτ between the dynamic and the static grass sliding cutting angles. The equations of grass sliding cutting angles of oblique rotary blades are also derived, and it is easy to see that the normal rotary tillage is only a special case of the oblique rotary tillage. Calculation result shows that the dynamic grass sliding cutting angle of sidelong edge of oblique rotary blade is smaller than that of normal blade, but the magnitude of dynamic grass sliding cutting angle of lengthwise edge changes of normal rotary blade. For both sidelong and lengthwise edge of the oblique rotary blade, the dynamic grass sliding cutting angle is always smaller than the static grass sliding cutting angle.