Optofluidic technique has an important application in cell analysis instrument. In view of the insufficient three-dimensional (3D) focusing ability of the trigeminal type-optofludic, the dynamic analysis of optofluidic with traditional structures is done, some factors affecting the 3D focusing are analyzed, and an efficient 3D focusing micro channel architecture is designed, which is suitable for the optical detection of white blood cells. The structure uses sheath stream flow’s press on both sides to achieve sample stream’s two-dimensional focusing. By adding the sags in the channel, it can achieve the sample stream’s 3D focusing. The focusing areas, velocity distribution and particle trajectory distribution under different kinds of sag are obtained with numerical simulation technique, and the corresponding 3D focusing effect is analyzed comparatively. Result shows that the proposed structure has a better 3D focusing ability and indicates that it will have a good potential application value for the optofluidic chip’s development and application in cytoanalyze.