Abstract: Mechanical quality factor Q_M is a key characteristic parameter of High-overtone bulk acoustic resonator (HBAR). The effects of structure parameter (thickness) and performance parameters (characteristic impedance and mechanical attenuation factor) of substrate, piezoelectric film and electrode composing a HBAR on Q_M are carried out firstly. The relationships between Q_M and these parameters are obtained by a lumped parameter equivalent circuit instead of distributed parameter equivalent circuit near the resonance frequency, and the analytical expressions of Q_M are given first. The results show that Q_M increases non-monotonically with the continuous increase of the substrate thickness for HBAR with certain piezoelectric film thickness, and it approaches to the mechanical quality factor of the substrate as the substrate thickness is large. Q_M decreases wavily with the continuous increase of the piezoelectric film thickness for HBAR with certain substrate thickness. Sapphire and YAG with low mechanical loss are appropriate as the substrate to get a larger Q_M. The electrode loss must be considered since it can reduce Q_M. Compared with Au electrode, A1 electrode with lower loss can obtain higher Q_M when the appropriate electrode thickness is selected. In addition, Q_M decreases with the increase of frequency. These results provide the theoretical basis for optimizing the parameters of HBAR and show that trade-offs between Q_M and K_eff² must be made in the design because their changes are often inconsistent.