Annular nozzle array is principally used in cylindrical laser.It contains two geometrical configurations of straight-throat style and circular-throat style.Both of them can be acquired by transforming linear nozzle array.Under conditions of constant gain volume,transverse dimension of annular nozzle array will reduced to 1/π of linear nozzle array.Thus annular nozzle array is especially suitable for the design of small-volume chemical laser.Annular nozzle array with circular throat is more applicable in actual chemical laser.Consequently flow field characteristics of annular nozzle unit with circular throat and linear nozzle unit were calculated based on three-dimension computational fluid dynamics methods.The numerical siumulation results demonstrate that Mach number and velocity magnitude in annular nozzle exit plane are higher by 4.03% and 0.356% as compared with linear nozzle under conditions of constant nozzle section dimension and throat area,whereas static temperature and static pressure of annular nozzle is lower by 6.08%，19.7%.Boundary-layer thicknesses of convergent section of two kinds of nozzles grow basically according to same trends and dimensions,nevertheless mean velocity magnitude of annular nozzle exit plane is faster than that of linear nozzle.As a result recombination proportion of F-atom is lower appreciably in annular nozzle exit plane than in linear nozzle exit plane.