To explore the yarn forming mechanism of dual-feed-opening (DFO) rotor-spun yarn, this work is based on the advanced movement of the peeling point and the merging effect of conventional rotor spinning. The fiber-layers distributions in the groove at the representative stages are characterized and analyzed in the form of illustration. Then the model of merging effect is deduced by mathematical induction. This model obtains the calculation formula of the merging number and the definition of the output-section. The morphological distribution of the original fiber-layer presents a nonlinear ladder like feature. The apparent color distribution of 1:1 blue/white DFO rotor-spun yarn theoretically shows the characteristics of two colors equally spaced, and the interval length is the length of the output-section. Using digital image processing tools, the length of output-section is verified by the image processing algorithm. This research has certain guiding significance for improving the theory of DFO rotor spinning unit (DFO-RSU).