A novel type of co-rotating non-twin screw extruder was presented in this paper. The speed ratio of the left screw element to the right one was two. In order to investigate the flow pattern and mixing mechanism in the flow channels, a novel visualization device was developed, which has the global transparent barrel. A kind of colored sodium carboxymethyl cellulose, which representes viscoelastic fluid, was used to examine the flow and mixing patterns. Four cameras were used to capture the instantaneous images during whole processing, and then the critical sizes of these images were obtained. A relational model of the axial length of fluid and the filling degree was established. The conductance method was adopted to measure the residence time distributions for different operation conditions, and the influence of the screw speeds on the material axial mixing was investigated. It is the first time that the flow picture of partially filled fluid in the non-twin screw elements is captured. The asymmetry phenomenon is found in the intermeshing region and the region far away from it. It is also found that the material dam is experienced elongation action in the single-tip screw channel.