We report a significant enhancement of the ultimate efficiency, η, of CH3NH3PbI3 (abbr.: MAPbI3) perovskite solar cells (PSCs) with optimized periodic poly(methyl methacrylate) (PMMA) nano texture, owing to the light trapping effect. With the finite difference time domain (FDTD) simulations, we investigated the η of the nano-textured PSCs (NT-PSCs) when the MAPbI3 layer thickness, dp, ranges from 31.25 nm to 2 μm. The optimized ultimate efficiency, ηm, is defined as the η optimized over the geometric parameters of the nano texture. When dp is approximately 125–500 nm, compared with that of the planar ones, ηm of the PSCs with optimized nano texture is significantly improved by ca. 11.5%–32.8%. Notably, when dp is 125 nm, ηm of the optimized NT-PSCs achieves 24.3%, which is relatively 32.8% larger than that of the planar PSCs. It is very potential to fabricate optimized PMMA nano texture in PSCs to enhance their PCE.