The invention relates to a cutting knife used for thermal cutting of plastic material. The cutting knife used for the thermal cutting of the plastic material comprises a blade body, and an inclined double-edged cutting edge which is arranged at the lower end of the blade body, the inclination angle alpha of the cutting edge relative to the horizontal line is 22-28 degrees, and the blade is grinded along the feeding direction of the blade by a grinding appliance to form the cutting edge. The cutting edge at least comprises a first level cutting edge and a second level cutting edge, the angle B1 of the first level cutting edge is 21-23 degrees, and the angle B2 of the second level cutting edge is 8-10 degrees. The plastic material cut by the blade is smooth in end face and free of the appearance of burs and silkings, and cutting end face quality of the material can be improved. Besides, in a cutting process, the resistance stressed on the blade is even, the blade is not prone to tipping, not only is the service life of the blade prolonged, but also cutting noise is reduced observably, and the production environment is improved.