Single-walled carbon nanotubes (SWNTs) were prepared by continuous CO_2 laser ablation at room temperature. Transmission electron microscope (TEM) and Raman spectrum of SWNTs samples prepared by laser with power from 500 W to 850 W were measured. It gave the dependence of growth condition and tube diameter on laser power, and confirmed that laser power play a crucial role in SWNTs formation and tube diameter distribution in the preparation method of long wavelength laser ablation. The yield of SWNTs increas with increasing CO2 laser power. On the other hand, the higher the laser power, the larger the tube diameter of SWNTs in experiments.