High-efficiency and ultrathin crystalline silicon solar cells (SCs) with a frequency upconversion (UC) layer and an array of silver nanohemispheres were presented. The light-trapping performances of SCs embedded with different volume ratios and radii of Ag nanohemispheres were systematically studied by finite-element analysis. The simulation results show that the short-circuit current density of the SCs and the light-field intensity in the UC layer can be significantly improved by adjusting the structural parameters of Ag nanohemispheres. The short-circuit current density of the structured SCs have been improved by 16.48% and the light-field intensity in the UC layer has been increased by 2.65 times compared to that of planar SCs. Additionally, the UC effects on the power conversion efficiency of the SCs were also investigated. The presented model will serve as the basis for further preparations of high-efficiency ultrathin crystalline SCs.