The present disclosure provides a cell separation apparatus for a bioreactor, which comprises: a cell separation component, which is arranged in a tank and immersed below the liquid level of the mixture and includes a filter membrane and a liquid guiding groove; and a tangential flow driving component, which includes one or more blades. The cell separation apparatus implements a low shear force and the filter membrane is not blocked easily. The present disclosure also provides a stirring system including a central shaft and a multi-layer paddle component. The multi-layer paddle component includes an upper paddle component, an intermediate paddle component and a bottom paddle component arranged around the central shaft.