Solar photocatalysis splitting water for hydrogen production is a very hot issue nowadays, and great expectations are laid on g-C3N4 due to its low-cost, thermal stability, low toxicity. However, pure g-C3N4 is not perfect and it still has some shortcomings. By using first-principles calculations, Br and Na are considered being co-doped into the g-C3N4 to get whose photocatalytic performance further enhanced. By studying the electronic structure, optical properties, we find that doping Br Na atom can obviously narrow the band gap, and promote the optical absorption characteristics of g-C3N4, which means a better performance in photocatalysis. Furthermore, the optical absorption cross section and optical scattering cross section of two typical morphologies nano particle and nano rode are calculated by optical simulations to analyze the effects of doping on the optical absorption and scattering characteristics, which could provide another view in analyzing photocatalysis mechanisms and can make great difference on future photocatalyst designing.