In this note we discuss the modifications to the existing beamline of Fermilab/NICADD Photoinjector Laboratory (FNPL) in order to improve the flat-beam emittance measurement system. The slit-method is used for the measurement, and the corresponding apparatus includes beam viewers and masks with slit. We found that it is necessary to replace the exiting multi-slit mask by single-slit masks due to the large emittance in the vertical plane. The distance between the slit and the beamlet-viewer downstream are optimized for
the measurement of both the large (in vertical plane) and small (in horizontal plane) emittance of the flat beam. We also present our study on the effect of the slit mask thickness, where we found that it is advantageous to using a 2~mm thick slit mask comparing to 6~mm as of the multi-slit mask.