The invention discloses slicon dioxide aerogel, as well as a continuous constant-pressure drying preparation method and a system thereof. The preparation method comprises the following steps of: using water glass, silicasol, silanol and the like which are relative low in price as silicon sources, uniformly mixing the silicon sources, water, a catalyst and a drying control agent to obtain a mixture, leaving gel to stand, crushing the wet gel into particles or powder, performing replacement with alcohol and liquid alkane, using a hydroghobing agent for modification, separating alkane gel, and finally, performing drying and sintering so as to obtain dewatering silicon dioxide aerogel. The silicon dioxide aerogel disclosed by the invention can realize continuous production, and is short in preparation period, small in pollution, low in cost and environment-friendly. The aerogel is powder or particles, the density is 0.04g/ml-0.3g/ml, the particle diameter is 0.1 mum-2mm, the specific surface is 500m<2>/g-1200m<2>/g, and the heat conductivity is 0.01W/mk-0.04W/mk.