Abstract: Using Reynolds-averaged Navier-Stokes(RANS) as governing equations,the axisymmetric co-axial jet flow fields were simulated and compared with single nozzle jets.Based on the change rule of the field parameters including the turbulent kinetic energy (k),velocity (v) and pressure (p),the co-axial jet flow field has been divided into joint section,primary section and main section.A detailed analysis of the characteristic and flow mechanism was also carried out herein.With analysis of the interaction between inner and outer nozzle jets,it indicates that the level of the turbulent kinetic energy inside the primary section is lower than that of a single jet,which may lead to the reduction of the jets noise;the level of the turbulent kinetic energy inside the main section is higher than that of a single jet,which may lead to the enhancement of the mixing between the jets and the ambient gas.At the end of this paper,numerical results of flow rate are compared to the test data,validating the algorithm used in this work.