Solar-pumped single- and multicore Nd3+-doped silica fiber lasers with primary concentrator were investigated using numerical simulation. Optimum length, maximum output power, and temperature distribution were calculated using the Monte-Carlo photon-tracing method. The highest laser output power of 20.36 W for 9-core fiber, at the optimal length of 260 m was obtained. Compared with a single-core fiber laser, the laser output power for 9-core fiber is increased by 1.61 times, and the optimal fiber length is reduced by 2.6 times. Solar-to-laser power conversion efficiency was equal to 2% which is the highest result obtained without any sensitizers for solar-pumped fiber lasers.