It is an important method of radar velocity measurement for measuring motion parameters in tubular structures. It can improve the accuracy and resolution of velocity measuring by improving the working frequency of radar. Based on the availability analysis of 122 GHz millimeter wave, the attenuation constant of pipe diameter and pipe wall roughness for millimeter wave transmission is established respectively. The simulation results show that the attenuation constant tends to decrease with the increasing of the pipe diameter. The attenuation constant tends to be stable when the tube wall roughness is larger than 5 μm; and the attenuation constant increases sharply when the tube wall roughness is less than 5 μm. This result further proves the feasibility of the speed measurement of 122 GHz millimeter wave in the tube and other tubular structures.