The invention provides a laser system of an oral cavity laser therapeutic instrument. The laser system comprises a laser power supply, a laser head, a laser water cooling system, an optical fiber output system, a gas-water spraying system and a complete machine control system, wherein the laser power supply is used for providing energy for the laser head, the laser head comprises a total reflection mirror, an output mirror, a xenon lamp, a laser rod, a light condensation cavity and an optical turn, the laser water cooling system is used for cooling the laser rod and the xenon lamp of a laser device, a light guide output system is used for laser output and is composed of a beam expanding shaping mirror, a coupling focusing mirror and an optical fiber, the gas-water spraying system is used for spraying a gas-water mixture to tooth tissue under the action of laser so as to achieve the effects of reducing the temperature of the tissue in an irradiation area and participating in laser treatment, and a central control system is used for controlling parameters of the laser power supply, the laser water cooling system and the gas-water spraying system and coordination work among the subsystems. The laser system has the advantages of stable output laser energy, good light spot uniformity, high operation flexibility and the like.