The simulation results between different facet reflectivity of external cavity and output power of distributed feedback fiber laser are presented, based on compound cavity laser model and facet reflectivity of external cavity measured by optical spectrum analyzers. And a two elements experiment system is constructed to prove this relationship. It is found that the output power of distributed feedback fiber laser will be increased and the flatness of distributed feedback fiber lasers array will be worsed due to the external optical feedback. And the larger facet reflectivity of external cavity is, the lower flatness of distributed feedback fiber lasers array will be. The influence of external optical feedback to the flatness of distributed feedback fiber lasers array should be considered, and distributed feedback fiber laser which have lower facet reflectivity should be used for constitutes the distributed feedback fiber lasers array.