Abstract: On-site laser additive repair of large titanium alloy parts is often difficult to accomplish in an atmospheric chamber due to limitations of conditions. The laser directed energy deposition tests of Ti-6Al-4V were carried out by using intra-optical powder feeding technology in a local atmosphere protection device under air-open environment and an atmosphere chamber with water and oxygen control functions, respectively, and the differences on the microstructure and mechanical properties of the Ti-6Al-4V alloy under the two forming environments were comparatively investigated. The results show that the average oxygen content of the formed parts in the open environment is higher than that of the formed parts in the atmosphere box. The microstructure of both types of formed specimens is mainly α+β dual-phase mesh basket structure, and the formed specimens in the atmosphere chamber have obvious β grain boundaries with lamellar α phases next to the grain boundaries.In contrast, there are a large number of α'phases within the grain boundaries of the formed specimens in the open environment.Higher cooling rate in the open environment and the intake of oxygen cause the strength of the alloy to increase and the plasticity to decrease. The tensile fracture of the specimen formed in the atmosphere chamber can be judged as microporous aggregation fracture, and that of the specimen formed in the open environment can be judged as transcrystalline fracture.