Abstract: The article discusses the use of parallel Mach probe to measure plasma flow velocity in deposit room of depositing diamond film device and the use of the fluid dynamics of Hutchinson to analyze the data.The results are:Mach number is 0.086,electron temperature is 5 eV,plasma flow velocity is 297 m/s.During the research of depositing deamond film,parallel Mach probe is a good tool for further study on the relation between plasma flow velocity and the quality and velocity of depositing diamond film.