The invention discloses a circumference ruler measurement device for length measurement of linear building materials and a large/small number transformation mechanism thereof. The circumference ruler measurement device comprises a circumference ruler line wheel, a large number ruler, a small number ruler and the large/small number transformation mechanism. The large/small number transformation mechanism comprises a large number ruler drive gear, a small number ruler drive gear and an engagement transmission device, wherein the engagement transmission device can be in engagement connection with one of the large number ruler drive gear or the small number ruler drive gear. The large/small number transformation mechanism adopts the engagement mode to realize connection and driving, thereby effectively preventing the skid phenomenon; and the circumference ruler measurement device adopting the large/small number transformation mechanism can accurately measure the length of the linear materials through the circumference ruler line wheel, and can trigger the next step when the linear material reaches a preset length. The whole process is finished mechanically and automatically; the advantage of full automatic control is realized; and the disadvantages that an electronic device is unstable and easy to damage are prevented.