Based on density functional theory, the single O2 and CO adsorption on pristine and palladium (Pd) doped graphene are studied using first-principles calculations. By calculating the system adsorption energy, charge transfer, band structure and density of states (DOS), we find that compared with O2 and CO adsorbed on the pristine graphene, the Pd doped systems have high adsorption energies and large charge transfers. The reason is that the new energy levels which are brought into pristine graphene by the dopant Pd strengthened the interaction between graphene and the adsorbed gas molecule. Oxidizing gas O2 and reducing gas CO have obviously different effects on band structure and DOS of graphene. The DOS near the Fermi level of graphene has great change after adsorbing O2 and the change becomes smaller when O2 is adsorbed on Pd doped graphene, while there is almost no change in DOS when graphene adsorbs CO, which indicates that doping Pd on graphene adsorbing CO will not enhance the gas sensitivity. However, the adsorption energy increases, which can improve the gas sensing response speed when graphene adsorbs reducing gas.