The invention relates to a gear internal diameter measurement mechanism based on self-centering and a measurement method thereof. The measurement mechanism comprises a centering driving portion, a passive rotation measuring portion and an equipment main body portion, wherein a body and a passive rotation sleeve are respectively mounted on a platen, the body is provided with a cylinder, a piston ofthe cylinder is connected with a lifting sliding table, a rotation fixing sleeve is mounted on the lifting sliding table, a power source is mounted on the rotation fixing sleeve through a fixing support, a rotation shaft is rotatably mounted on the rotation fixing sleeve, one end is connected with an output end of the power source, the other end is connected with a positioning top tip, a rotationdisc and a rotation seat are arranged below the positioning top tip, the rotation seat is rotatably mounted on the passive rotation sleeve, the rotation disc is mounted on the rotation seat and is used for carrying a to-be-measured gear, and a gauge block type displacement sensor is arranged in the rotation seat and is mounted on the passive rotation sleeve through a sensor support. The measurement mechanism is advantaged in that automatic centering, active pressure driving, passive rotation and dynamic measurement are realized, the measurement mechanism is suitable for measuring gears in multiple internal diameter dimensions, measurement precision is high, and work reliability is realized.