The upward and downward motions of chromospheric plasma, which are called chromospheric evaporation and condensation, can be observed during solar flares. The Mg II lines of IRIS provide a good opportunity to study the evolution of chromospheric plasma. Here we particularly focus on the emission property of Mg II triplet line 2791.6 Å in the 2014/10/27 M7.1 flare. To understand the behaviors of Mg II lines, two localized areas with flare brightening feature are selected for analysis. The Mg II h and k lines are also used for comparison. By combining with IRIS, GOES, and RHESSI observations, we find that the 2791.6 Å has different evolution profiles at these two areas. Possible explanations will be discussed in this study.