By adjusting the ratio of SiO₂ /RO (R=Ca, Mg), this article studied the atomic structure of CaO-MgO-Al₂O₃-SiO₂ glassy melts by a molecular dynamics simulation. The Pair distribution function (PDF), Coordination number (CN), bridging oxygen (BO), non-bridging oxygen (NBO), and Qn were utilised to characterise the glass structure. The results indicated that the content of the bridging oxygen was decreased with the RO replacing the SiO2. The total content of Q⁴ in the tetrahedron changed significantly from 34.11% to 18.06%. Meanwhile, the NBO/T parameter increased from 1.298 to 1.555. In the corresponding viscosity test, the melting temperature, fitted by the MYEGA equation, also decreased with the decrease in the SiO₂ content, and the fragility value increased from 39.12 to 53.20. Finally, the fragility and NBO/T were linked to describing the relationship between the mid-range structure and the rheological property. Moreover, the analyses showed that they had a linear relationship.