We have investigated the coherence of resonance fluorescence for a coherently driven two two-level atoms coupled to a single-mode cavity in the bad cavity limit. By adjusting the detuning of the cavity-mode frequency from the driving filed frequency, we can enhance the fringe contrast factor as well as decrease the second order correlation function. The surprising effect produced by the detuning has an effect on the atomic dress-state population, which makes the fringe contrast factor and the second order correlation function changed evidently.