To improve the performances of thin-film solar cells, particles swarm optimization and the finite-difference time domain algorithm are combined to optimize the structural parameters of the metal nanogratings on the surface of solar cells. A test for the short circuit current over the solar spectrum shows an enhancement up to 39% when comparing with bare thin film cell. Absorption improvement and quantum efficiency enhacement in different polarization mode are analysed. Analytical results show that, the guided-wave modes coupled in the absorbing layer by metal nanogratings, surface plasmon polaritons supported by metal nanogratings and Fabri Pérot resonance in the absorbing layer lead to the performance improvement in thin-film solar cells.