The invention discloses a BZ original number tailoring method of a clothing chest expanding piece. Clothing original numbers of the chest circumference, the waistline, the waist line height, the shoulder width, the collar length, the armhole and the clothing length are determined according to human body measurement data and predetermined clothing styles. A front collar length line, a rear collar length line, a shoulder line, a front armhole line and a rear armhole line are drawn on folded fabric, a waist contracting point, a bottom edge line and a side sewing line are determined, and a front and rear clothing piece mother board for sewing and tailoring is arranged on the side sewing line, the rear armhole line, the shoulder line, the rear collar length line and the bottom edge line. A chest upper piece dividing line and a chest lower piece dividing line are determined on the mother board, a chest expanding amount line and a chest rising amount line are drawn on a chest piece according to the chest expanding amount and the chest rising amount, and therefore tailoring of the chest expanding piece forming board is achieved. The BZ original tailoring method of the clothing chest expanding piece is fast in drawing and tailoring, capable of meeting the requirement for the changing precision and speed of clothing of a clothing market, simple, practical and convenient to use, and the drawn and tailored clothing is high in comfort degree.