In this study, nanosilica particles were prepared from self-made silica gel under the protection of citric acid in situ complexation and ethyl acetate as the latent acid reagent. The effects of ethyl acetate, citric acid, reaction temperature, and reaction time on the particle size of nanosilica were studied. The optimum process parameters were determined as follows: V ethyl acetate : V silica sol  = 9:50, citric acid addition amount 3.64%, reaction time 8 h, and reaction temperature 70 °C. The products were characterized by IR, XRD, SEM and TEM analyses. The results show that the nano-silica particles prepared by complexation precipitation method employing silica sol as silicon source has good dispersion with average particle size controlled about 19.44 nm. The method has numerous advantages over the conventional inverse extraction method under colloid protection, and is expected to achieve industrial production.