Based on the fluctuation of light, a spiral nanoantenna structure for solar energy harvesting is proposed and analyzed using finite difference time-domain method. The nanoantenna structure consists of two coplanar Archimedes spiral arms and a substrate layer as supporting. The radiation efficiency and polarization characteristics of the nanoantenna are analyzed. The new spiral nanoantenna achieves a total radiation efficiency of 74.49% within the wavelength range from 400 nm to 1600 nm (takes up 85% of the total solar energy), which is higher than that of dipole antenna structure proposed by previous researchers. Meanwhile, for linearly polarized plane waves from different directions, local electric field enhancement with the same level at the spiral antenna feed gap can be obtained, which indicates that the antenna can process arbitrary polarization characteristics of sunlight.