This article describes the study of the mechanism of fiber microformation in melt blowing. Through theory analysis of the flow field of molten polymers, it is inferred that the molten polymer flows in the form of the collective motion as a unit of microlayer. There is a sliding motion between every two adjacent microlayer units, and the attenuation mechanism of the fiber formation involves different microlayers that are arranged in a longer length than that before the fiber is drawn along the axial direction in terms of their velocities. Melt blowing experiments and comparison experiments were designed and performed. After measurements of the fibers and the analysis of the experimental data, results were obtained. These results verify the inference from the principle of the shear flow and the mechanism of the fiber microformation. When the theory of the formation mechanism is used 6:5 explain the fiber tenacity, by inference the tenacity of MB fiber is lower than that of spunbond or melt spinning fiber.