A terahertz （THz） spectrometer plays an essential role in THz astronomy. Coplanar waveguides （CPWs） are critical components in such THz spectrometers and relative permittivity is one of the most important parameters of a dielectric material. It decides the resonant frequency and the quality factor of the device. Since accurate electromagnetic field solutions of a CPW are hard to get due to its complex structure， the conformal mapping technique （CMT） is widely used to get approximative analytical expressions for effective permittivity . However， it makes a difference when a thin-film dielectric is involved. In this paper， we utilized high frequency structural simulator （HFSS） to get the effective permittivity of CPW based on resonator structure and compared it with that from conformal mapping technique. Provide the possibility to characterize the thin-film dielectric through simulation.