Microcrack propagation in asphalt mixtures leads to macroscopic cracks in pavement. It is necessary to study crack evolution from the perspective of micromechanics to precisely predict structural damage. This paper developed a micromechanical model of three-point bending under a bending fatigue load for a bitumen specimen that was a three-phase asphalt mixture composed of coarse aggregate, asphalt mortar and air voids. A three-point bending test was carried out to verify the correctness of the micromechanical model. The microscopic parameters of bitumen were calibrated by fitting the stress-strain curve of the laboratory test data. Microdynamic simulations demonstrated that the growth of microcracks included an initial transition period, a rapid growth period and a stable period. The microcrack propagated from the bottom edge to the center of the specimen under the bending load, mainly within the asphalt mortar or asphalt-aggregate interface. Compared with a static load, the longitudinal stress at the microcracks was much larger, and the number of microcracks increased more rapidly, under the dynamic load. The width of the microcrack increased instantaneously when the internal stress state of the specimen reached its material strength, then changed almost periodically with sinusoidal loading and finally stabilized. The micromechanical modeling method can potentially be used to predict the position and width of macroscopic cracks in asphalt pavement.