Light trapping is of critical importance for constructing high efficiency solar cells. In this paper, we first reviewed the progress we made on the plasmonic light trapping on Si wafer solar cells, including Al nanoparticle (NP)/SiNx hybrid plasmonic antireflection and the Ag NP light trapping for the long-wavelength light in ultrathin Si wafer solar cells. Then we numerically explored the maximum light absorption enhancement by a square array of Ag NPs located at the rear side of ultrathin solar cells with wavelength-scale Si thickness. Huge absorption enhancement is achieved at particular long wavelengths due to the excitation of the plasmon-coupled guided resonances. The photocurrent generated in 100 nm thick Si layers is 6.8 mA/cm2, representing an enhancement up to 92% when compared with that (3.55 mA/cm2) of the solar cells without the Ag NPs. This study provides the insights of plasmonic light trapping for ultrathin solar cells with wavelength-scale Si thickness.