Micro porosity is the main issue in wire arc additive manufacturing （WAAM） of 2219 aluminum alloy. In this study， the effect of process parameters on the porosity characteristic of WAAM 2219 aluminum alloy was quantitatively investigated. It is found that the porosity was influenced by the heat input. When the heat input is low， the porosity increases with the increasing of heat input due to the nucleating and growing of pores. When the heat input is high， the porosity decreases with the increasing of heat input due to the escaping of pores. The porosity is below 0.2% when the heat input is as low as 230.5 J/mm or as high as 439.5 J/mm.