NOVELTY - Open-air pit type forced carbonization kiln comprises kiln body, kiln bottom, purifying system and smoke discharge system. The smoke discharge system comprises gas inlet, smoke outlet, powerful draft fan and chimney. The purifying system is a main smoke discharge purifying passage. The kiln body (1) is open-air pit type. The smoke discharge system further comprises a kiln bottom smoke discharge system connected with the gas inlet, the smoke outlet and the main smoke discharge purifying passage. The kiln bottom smoke discharge system comprises a transverse kiln bottom smoke discharge groove.    USE - Used as an open-air pit type forced carbonization kiln.    ADVANTAGE - The kiln ensures large single-kiln volume, uniform carbonizing and low carbonizing cost, and hence solves the problems of high cost of the existing carbonization kilns for carbonizing the red willow shrub, small single-kiln capacity and long carbonization period.    DETAILED DESCRIPTION - Open-air pit type forced carbonization kiln comprises a kiln body, a kiln bottom, a purifying system and a smoke discharge system. The smoke discharge system comprises a gas inlet, a smoke outlet, a powerful draft fan and a chimney. The purifying system is a main smoke discharge purifying passage. The kiln body (1) is open-air pit type. The smoke discharge system further comprises a kiln bottom smoke discharge system connected with the gas inlet, the smoke outlet and the main smoke discharge purifying passage. The kiln bottom smoke discharge system comprises a transverse kiln bottom smoke discharge groove, a vertical kiln bottom smoke discharge groove, a flue wall body (10) and a vertical flue wall body smoke discharge passage (11). The kiln bottom (2) is made up of 9-13 rows of fireproof bricks. The vertical kiln bottom smoke discharge groove is mounted between each row of the fireproof bricks along the longitudinal direction. The transverse kiln bottom smoke discharge groove is mounted between the adjacent fireproof bricks along the transverse direction. An INDEPENDENT CLAIM is also included for producing charcoal using the open-air pit type forced carbonization kiln, comprising: (a) grading the red willow shrub branches according to the thickness, cutting the red willow shrub branches using red willow shrub branches cutting machine of diameter less than 3 cm into short sections of 3 cm, placing the short sections of the red willow in the open-air pit type forced carbonization kiln for carbonizing, cutting the thick red willow branches of diameter greater than 3 cm into sections of 20 cm, and directly placing in the carbonization kiln; and (b) igniting the red willow shrub branches in the carbonization kiln from upper side, forcibly combusting the red willow shrub branches under anoxic state with the effect of the spontaneous combustion of the red willow shrub branches, the smoke discharge system and the draft fan at the ground; where the carbonizing process comprises drying, carbonizing and cooling, and the red willow shrub branch is dried at 80-200 degrees C for 3-6 hours; in carbonizing process, the temperature of the red willow shrub branches is gradually increased to 600 degrees C from 200 degrees C in 20-24 hours; and in the cooling process, the are branches discharged when temperature is 25-45 degrees C within 12-18 hours.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of open-air pit type forced carbonization kiln.    Kiln body (1)    Kiln bottom (2)    Flue wall body (10)    Vertical flue wall body smoke discharge passage (11)    Flue serial hole (12)