A modified micromixer design based on the concept of multidirectional vortices and unbalanced splitting and recombining of fluid streams is described in this work. The purpose of this study was to demonstrate that the combination of unbalanced inertial collisions, multidirectional vortices, and converging/diverging flow caused by the fan-shaped cavities contributes to the improvement of the mixing effectiveness. By using computational fluid dynamics (CFD), numerous studies of the mixing performance and flow features were completed at moderately low Reynolds numbers ranging from 1 to 80. Experimental analysis of the mass transfer and mixing quality along this modified micromixer was performed to validate the numerical results. The computational and experimental results for the concentration distributions and flow patterns reveal the following trends: (i) The combination of transverse Dean vortices effect (vertical plane) and expansion vortices (horizontal plane) effectively contribute to the considerable improvement of the mixing performance. The experimental results were consistent with the numerical results in this respect. (ii) The geometric parameters and the arrangement of the fan-shaped cavity structure are two important factors affecting the mixing performance. When the fan-shaped cavity channel is 3 times as wide as the major subchannel, the mixing index of this type of micromixer was found to reach around 7596 at Reynolds numbers larger than 60. Furthermore, we also analyzed the relation between mixing efficiency and pressure drop simultaneously.