By using the first-principles, the electronic structure and optical properties of C-N point defects on sidewall surface of [0 0 01]-oriented GaN nanowires were calculated. We found that C-N point defects are more likely to be enriched on the surface of the GaN NWs. In addition, the closer the C-N point defect to the sidewall of the GaN NWs, the greater the effects of C-N point defect to the top of valence band. The C-N point defects in different depths may lead to the yellow luminescence, and the red shift of PL spectral wavelength would occur when it tends to the surface. This may be one of the reasons for the broadening of the yellow luminescence.