The invention discloses a detection apparatus of a rail train wheel tread rolling circle diameter, and a detection method thereof. The detection apparatus comprises a detection ruler arranged on the inner rim surface of a wheel, one end of the detection ruler is provided with a fixed detection block, the other end of the detection ruler is provided with a movable detection block, the fixed detection block and the movable detection block are in contact with a position distanced a set height away from the inner rim surface on the wheel for detecting the wheel tread rolling circle diameter. The detection method realizes detection of the wheel tread rolling circle diameter by use of the detection apparatus of the rail train wheel tread rolling circle diameter. According to the invention, the diameter dimension of the tread rolling circle diameter of a vehicle finished product can be detected, the circularity of a tread rolling circle can be detected, the detection is convenient, and the detection accuracy is high.