This study constructed an ultrathin photovoltaic cell model with an inverted pyramid velvet surface front structure, double-layer anti-reflection film, 3 μm thick single-crystal silicon absorption layer, finger-crossed back contact electrode, and oxide passivation layer. The study first used the finite-difference time-domain (FDTD) method to solve the Maxwell equations of electromagnetic wave propagation in semiconductors, and it then obtained the field distribution. Next, the inclination of the inverted pyramid velvet surface was optimized and an optimal short-circuit current density of 44.65 mA?cm-2 was achieved. The study used small-signal alternating current scanning to solve the drift-diffusion and Poisson equations describing the carrier motion law and electrostatic potential, respectively. A particle swarm optimization algorithm was then employed to optimize the position and doping concentration of the finger-crossed back contact electrode. The model's volt-ampere characteristic curve was obtained, and its photoelectric conversion efficiency was calculated to be 23.03%.