Buffer gas cooling is the most effective and practical method to cool ions in ion trap. The kind and quantity of buffer gas are the key technologies in mercury ion microwave frequency standard experiments. Buffer gas made of helium, neon or argon was studied to cool trapped mercury ions (199Hg + ) in a linear ion trap by introducing a resistance term in Mathieu’s equation. It’s found that the decay time of motion of 199Hg + in argon gas is the shortest, and the frequency shift of the clock transition (40.5 GHz) is minimum when the pressure of helium is 10-5 Torr or the pressure of neon gas is 2.4×10－5 Torr. Neon is the most suitable buffer gas among helium, neon and argon, considering the decay time-constant of motion of 199Hg + in buffer gases and 199Hg + clock transition shift’s sensitivity to the change of the pressure of buffer gas.