The acid-base ratio and Si-Al ratio of coal ash are the key factors affecting coal ash properties. Adding additives to coal or using mixed coal to adjust coal ash fusing characteristics by changing the ratio of SiO2 to Al2O3 is an important means to improve the ash and slag problems in the high temperature coal conversion process. In order to analyze the influence of the change of Si-Al ratio on the microstructure of coal ash system and obtain the theoretical mechanism of the change of macroscopic properties，the radial distribution function (RDF)，mean coordination number (CN)，degree of polymerization，distribution of oxygen and other microstructures in CaO-SiO2-Al2O3 system were simulated and calculated when SiO2/Al2O3 increased from 0.5 to 3.1 based on first principles at 1 600 ℃. The results show that：when SiO2/Al2O3 increases from 0.5 to 3.1，Si—O has a stable four coordination structure and the average bond length is 1.61×10-10 m; the average bond length of Al—O is 1.75×10-10 m，the coordination number of Al—O is relatively stable，and the coordination number decreases with the increase of Si-Al ratio. Si and Al are network formers in the silicate system of synthetic coal ash. As SiO2/Al2O3 increases from 0.5 to 3.1，Q4 and Q5decrease gradually，forming Q and Q3 with low degree of polymerization. Among them，Q4 and Q5decrease by about 23% and 10% respectively and Q3 and Q increase by about 14% and 11% respectively. A large number of ［SiO4］4- and ［AlO4］5-are destroyed，the degree of polymerization is reduced，the disorder is increased，and the network structure is destroyed，resulting in the decrease of ash fusibility. With the increase of Si-Al ratio，Ot and Ob decrease gradually，and Onb increases significantly. Among them，Ob and Ot decrease by about 7% and 13% respectively，and Onb increases by about 18%. The network structure of the system is destroyed，and the loose network structure makes the system more prone to collapse at high temperature. The macro performance is that the melting temperature of coal ash is reduced，and it is easier to produce low-temperature eutectic.