The thickness of the active layer is limited by its low carrier mobility in the polymer solar cell composed of the blend bulk-heterojunction formed by P3HT as donor material and PCBM as acceptor material, which can affect the light absorption in the polymer solar cell. Nano-structure gratings inserted into polymer layer can redistribute the electrical field inside the device and improve its light absorption. Two-dimensional electrical field distributions inside the polymer solar cell are simulated with the grating period of 1 μ, fill ratio of 0.5 and incident wavelengths of 500 nm and 700 nm based on finite difference time domain. The light absorptions by the devices with different grating depths and fill ratios are calculated based on rigorous coupled wave. The analysis illustrates that light spots occur in the device due to the light diffraction caused by the gratings and the light absorption is increased by 4.2% with a grating fill ratio of 0.5, depth of 10 nm and an incident light wavelength of 512 nm. In experiment, nano-structure gratings are introduced into the devices by the micro-printing technology with PDMS and polymer solar cell is structured with ITO/ PEDOT:PSS gratings/ P3HT:PCBM/ LiF/ Al. The experimental results from the planar and the grating devices prove that the nano-structure gratings embedded in PEDOT:PSS layer increase the power conversion efficiency by 31%.