Using ascorbic acid as a reducing agent, AgNO3 as silver salts and polyethylene glycol 4000(PEG-4000) as dispersant, sub-micron spherical silver powders were prepared by a liquid-phase reduction method. By means of the orthogonal design of three factors and three levels, the effects of the following factors on the particles size of silver powders were studied, including the concentration of ascorbic acid, the way of dispersant addition into solution and the pH value of the solution. The optimized preparation conditions of sub-micron spherical silver powders were obtained. The sub-micron spherical silver powders could be synthesized under the conditions of pH=5, AgNO3 concentration of 0.5 mol/L, adding half of the dispersant to the AgNO3 and ascorbic acid solution, C6H8O6 concentration of 0.9 mol/L, PEG-4000/AgNO3 mass ratio of 1.2 and at room temperature. The sub-micron spherical silver powders were characterized by laser particle distribution analyzer, SEM and XRD.The results show that the prepared silver powders are pure, with uniform particle size distribution and spherical shape under optimized experiment conditions. The paste prepared with the obtained silver powders was printed on the silicon, and the square resistance of the sintered film is 0.721 mΩ/□, indicating the electrical property of the prepared silver is qualified for solar cells.