The invention discloses a laser detector, including the machine body, the laser detection head, the laser controller, the level mobile device, and the fixture part, and the said laser detection head and the laser controller are installed in the machine body, the level mobile device installed between the laser transmitter and the laser receiver on the main supporting board of the machine body, and the fixture part connecting with the level mobile device, which can left and right move with the driven of the level mobile device, and the fixture part has the rotating pair rotors, the groove formed between the rotors is used to place the detected products. The laser diameter detector also includes the control device to control the said laser detection head, the laser controller, the level mobile device, and movement of the fixture part. The invention can detect the diameter and beating value of the shaft units, with high detection speed and accuracy, to solve the difficulty the traditional manual detection, which is slow and low accuracy, and it has the multi-function detection, flexible parameter setting, easy and convenient operation, and this invention can meet the great demand for OA production industry, with the broad market and higher economic benefit.