In order to study the optimum atmosphere condition for preparing waste tea-based activated carbon by H3PO4 activation, activated carbons were prepared under nitrogen (NAC), air (AAC) and steam (SAC) atmospheres, respectively. The physicochemical properties of the three activated carbons were characterized by N2 adsorption/desorption and Fourier transform infrared spectroscopy (FTIR). AAC exhibited highest values of the Brunauer–Emmett–Teller (BET) surface area (880m2/g) and total pore volume (0.680cm3/g) than that of NAC (824cm2/g and 0.666cm3/g) and SAC (785cm2/g and 0.629cm3/g). Nevertheless, the adsorption results for oxytetracycline (OTC) showed that SAC had the strongest adsorption capacity than NAC and AAC. The adsorption equilibrium data for OTC adsorption onto the three activated carbons agreed well with the Freundlich model, and the adsorption kinetics were well fitted in pseudo-second-order model. Results revealed that activated carbon prepared under air atmosphere showed the largest BET surface area and activated carbon prepared under steam atmosphere showed the best adsorption capacity.