Silica sols were prepared by the sol-gel method using ethyl orthosilicate (TEOS) as raw material and ammonia as catalyst. The effect of ammonia addition on the particle size as well as the stability of silica sols were studied through SEM-EDS, XRD, thermogravimetric analysis, laser particle size analysis, Zeta potential, and other analytical means. The results show that when the pH value is in the range of 11~12 and the molar ratio of ammonia to ethyl orthosilicate R(n(NH3·H2O)∶n(TEOS)) is in the range of 1~10, the average particle size y of silica sol is index correlated with the R value x. The fitted function is y=2.22x1.79 with a correlation coefficient of 0.96, and the particle size increases from 1017 nm (R=1) to 142.48 nm (R=10), and the half-height width of the particle size distribution increases from 9.89 nm (R=1) to 171.61 nm (R=10). The stability of the silica sol shows a downward parabolic trend with the addition of ammonia, with the gel time decreasing from 684 h (R=1) to 28 h (R=5) and then increasing to 780 h (R=10).