The modification of multi-walled carbon nanotubes on the surface of silicon solar cells without antireflection layer is studied. The modification craft is realized by dropping uniform mixture of multi-walled carbon nanotubes and ethanol on the surface of polycrystalline silicon solar cells. The short-circuit current increases by 5.61%, but open circuit voltage, maximum output power and the fill factor decreases by 4.36%, 6.15% and 7.11% respectively when the modification reaches the best state. For polycrystalline silicon solar cell, both advantages and disadvantages of the modification of multi-walled carbon nanotubes exist, and this indicates the existence of optimization point of modification. With PC1D simulation software, the principles of modification are analyzed from aspects of theoretical and numerical analyses, and the reasons for the change of electrical parameters are explained. Through experiments and analyses, a way of reducing the concentration of multi-walled carbon nanotubes is proposed to optimize the effect of modification.