A new horizontal neutron reflectometer has been designed and now under construction at the HANARO, 30 MW research reactor, in Daejon, Korea. We performed simulations of neutron ray-tracing to evaluate the performance of all of the optical components of the instrument with a Monte Carlo technique using McStas. The feasible wavelength of the incident neutron beam is $2.5{AA}$. It produces a q-range up to $0.126{AA}^{-1}$ with a supermirror as a deflector. Our studies showed improvement of the performance of the guide tube and monochromators. Although the performance is limited in q-range, it promises to be the first reflectometer in Korea for the study of free surfaces, which is currently in demand.