The COSINE-100 experiment is a joint experiment by the KIMS-NaI (Korean Invisible Mass Search) and the DM-Ice with a goal to reproduce the annual modulation signature reported by the DAMA using the same target material. In rare event searches, the cosmic-ray muon can produce the seasonal modulation of events which can mimic weakly interacting massive particles (WIMP)-like signal in nuclear recoils. To tag the muon-events and study the correlations between muon and crystal signals, COSINE-100 has installed muon detectors in the shielding structure at Yangyang Underground Laboratory (Y2L). An array of 3-cm thick plastic scintillator panels surrounds the crystal detector in all sides. This paper describes the assembly of the detector, DAQ, test performance, and the muon flux measured at the experimental site. The detector efficiency is calculated to be 99 $\pm$ 1% and preliminary result of the muon flux is found to be 344 $\pm$ $1{(stat.)}$ $\pm$ $34{(syst.)}$ muons/m$^2$/day.