The invention discloses an oxygen concentration detector, which belongs to the technical field of gas concentration detection. The invention aims to solve the problems that the prior oxygen detector based on spectrum technology has high cost due to using a single-mode diode laser and the output wavelength is sensitive to temperature change. An output end of a signal generator of the oxygen concentration detector is connected with an input end of a drive unit; an output end of the drive is connected with an input end of a tunable multimode diode laser; output light of the tunable multimode diode laser is transmitted by a collimating lens to form parallel light; the parallel light is split into transmitted light and reflected light through a spectroscope; the transmitted light passes through a sample cell, then is incident onto a first focusing lens for focusing and then incident onto a first photoelectric detector; the reflected light is taken as reference light and is incident onto a second photoelectric detector at last; two paths of light signals are converted into electric signals and then are input into a computer through an A/D converter; medium in the sample cell is oxygen to be tested; and the central wavelength of the tunable multimode diode laser is 760nm. The oxygen concentration detector is used for monitoring oxygen concentration.