The black silicon is fabricated by the metal assited chemical etching method. The rounding effect of the ultra low concentration of NaOH solution on the structure of black silicon is studied, as well as on the performance of the black silicon solar cell. Scanning electron microscope(SEM) and quantum efficiency(QE) tester are used to analyze the structure of black silicon and the performance of black silicon solar cell. The results show that the ultra low concentration of NaOH solution can remove the edges and cusps of black silicon with the widened nanostructure in order to reduce the surface recombination. It is also found that with the NaOH treatment, the nanostructure turns out to be smooth and uniform. The average reflectance of black silicon solar cell is 4.15% in the wavelength range of 400—900 nm. And the average efficiency of black silicon solar cells achieves 17.94%, which is relatively 0.35% higher than that of silicon solar cells fabricated by the traditional acid textured method.