Turbulent combustion remains to be one of most complicated technologies due to the complexities of turbulence and combustion as well as the interaction of both. This paper presents a vitiated coflow combustor, which is newly used for the fundamental research into turbulent combustion. The characteristics of controllable active thermo-atmosphere (CATA) of a vitiated coflow combustor are investigated. The results show that the oxygen mole fraction of vitiated coflow flames between 0% and 21% yield coflow temperature between 700 and 1500 K, and there is a constant temperature space as a cylinder with a radius of 40 mm. These features of the vitiated coflow indicate the existence of a controllable active thermo-atmosphere, which benefits the basic study on the autoignition of a combustible mixture in a homogeneous charge compression ignition (HCCI) combustion.