The invention discloses an online sharpening device comprising a spindle support (1), planer blades (2), a planer tool (3), a spindle (4), a spindle auxiliary support unit (5), a tool guard and sharpener holder (6), a sharpener (7), and a sharpener guide rail (8) disposed inside the tool guard and sharpener holder (6). The sharpener (7) is slidably disposed on the sharpener guide rail (8). A drive cylinder (9) is fixed on the spindle support (1). A piston rod of the drive cylinder (9) is fixedly connected with the sharpener (7). The online sharpening device has the advantages that online automatic sharpening of planer blades is achieved, time is saved, repeatedly clamping the blades is not required, the service lives of the blades are prolonged, sharpness is kept anytime, cutting performance is greatly improved, machining efficiency is improved, the speed of the spindle need not be very high during high-speed cutting, and good surface quality of workpieces can be ensured.