The effects of mass concentration of silica fume solution, ultrasonic dispersing time, ultrasonic dispersing frequency, dispersing medium and test precision of laser particle size analyzer(LSA) on particle size measurement of silica fume were investigated using median particle diameters(D50) as evaluation indicator. The correlation of specific surface area of silica fume measured by laser particle size analyzer and gas adsorption using the BET method was studied. The results indicate that the laser particle size analyzer with high accuracy(test range 0.022000.00μm) is applicable to particle size measurement of silica fume and it has high degree of reproducibility. The dispersion effect of water is better than that of alcohol. When the mass concentration of silica fume solution is 515g/L using water as dispersing medium, the ultrasonic dispersing frequency is higher than 40kHz and the ultrasonic dispersing time is longer than 10min, the optimum dispersion effect of silica fume can be achieved. There are some correlation between silica fume specific surface area measured by laser particle size analyzer and that by gas adsorption using the BET method, but the former is smaller than the latter.