Ag nanowires (Ag NWs) are synthesized by polyol method under the conditions of different temperature of reaction solution, different addition amount and injection rate of polyvinylpyrrolidone (PVP). The structure and the process of lateral growth of Ag NWs were observed and analyzed by X-ray diffraction (XRD), ultraviolet-visible absorption spectrum (UV-VIS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It showed that the lateral growth of Ag NWs and longitudinal growth of Ag NWs occurrs at the same time by UV-VIS. And in the later stage of synthesis of Ag NWs, the peak in UV-VIS, which indicated the lateral growth of Ag NWs, red-shifted obviously from 384 nm to 388 nm. This rapid redshift implied that the diameters of Ag NWs increased quickly. In other words, rapid lateral growth of Ag NWs occurred in the later stage of synthesis of Ag NWs. According to the SEM of Ag NWs, in the early stage of the reaction (15~23 min), the diameter of Ag NWs increased by only 20 nm, but in the later stage of reaction (23~30 min), the diameter of Ag NWs increased by nearly 150 nm. The result of SEM observation is consistent with the analysis of UV-VIS. It was also found that the lateral growth of Ag NWs is related not only to the sizes of seeds but also to the thicknesses of the outer Ag layers. Tiny Ag particles with diameters of several nanometers adsorbed onto the side facets of Ag NWs and acted as adsorption points for Ag source. The lateral growth of Ag NWs was caused by the continuous multipoint adsorption of Ag source on the side of Ag NWs. Decreasing the temperature of the reaction solution (from 165 to 155 degree), reducing the injection rate (from 67 to 49 mL·h-1) and the addition amount of PVP in the later stage could inhibit the lateral growth of Ag NWs and increase the aspect ratios of Ag NWs remarkably. The diameters of Ag NWs decreased from 200 nm to 100 nm, but their lengths still maintained above 100 μm.