Ar-circulated hydrogen engine replaces atmospheric nitrogen with Ar so as to increase engine efficiency and avoid NOx forming. Due to the differences in gas molecular weight of injection gas and ambient gas, two injection strategies were proposed for this engine. This study aims at measuring the characteristics of non-reactive H-2 and O-2 jets in Ar atmosphere by Schlieren method. The experimental results demonstrate that the increase of injection pressure or the decrease of ambient pressure leads to an increase of penetration. Jet penetration length of O-2 is slightly lower than H-2 by 0-6%. The dispersion angle of gas jet increases slightly with the increase of injection pressure and ambient pressure. Under all the experimental pressure boundary conditions, the dispersion angles of H-2 jets are between 31 degrees and 36 degrees and the dispersion angles of O-2 jets are between 22 degrees and 28 degrees. The entrainment rate of gas jet increases with the increase of injection pressure, ambient pressure and injection time. Jet entrainment rate of O-2 is 2.43-2.98 times that of H-2 jet because of the lower volume flow rate of O-2 jet. H-2 jet entrains more total argon volume for its higher penetration and dispersion angle. The actual engine prefers to use the hydrogen injection strategy.