Atmospheric He+O2 cold plasma, rich in reactive oxygen species(ROS), has attracted a great deal of attention. Much research has focused on the ROS generation in plasmas, but little for delivering of such species out of plasmas. In plasma treatment, the dosage of ROS that can act on the treated target is of great importance, but how to quantify such dosage by simulation is still unknown. Regarding to this, a fluid model of He+O2 radio-frequency(RF) discharge is developed in this paper, by which the dosage of ROS is quantitatively investigated. It is found that the dosage of ROS is dominated by plasma sheath, because the ROS generated in bulk regime has little probability to be delivered out of plasmas and act on the treated target.