Nano-silver has received a great deal of attention in recent years for its various potential applications on medical, electronic, optical, etc. In this paper, we developed a simple way to synthesize nano-sized silver by liquid chemical reduction method with hydrazine hydrate as reductant and PVP as surface-protection reagent. In the experimental design, orthogonal test with three factors and three levels is adopted, which investigated the effect of instillation method, the PVP amount and the reaction temperature on the size and morphology of silver nano-particle. Through analyzing the orthogonal experimental results, the optimum conditions for nano-silver preparation is obtained, in which hydrazine hydrate solution is instilled into the mixture solution of silver nitrate and PVP, the molar ratio of PVP/AgNO3 is 1.5, and the temperature is 50°C. Scanning electron microscopy was used to characterize the morphology of silver nano-particles, and the results show that the dispersed spherical nano-particles with the average size of about 60 nm can be prepared in this work.