For traditional machine, to modify spiral bevel gear tooth-profile lengthwise curvature, cutter must be disassembled and adjusted its diameter. This is a complicated process. On domestic-made 4-axis CNC spiral bevel gear machine, machining coordinates were set up, cradle radial setting and roll ratio polynomials were proposed. With tooth contact analysis (TCA) technology, considering contact lines and transmission errors, the polynomials were taken into TCA process to study lengthwise curvature correcting rules. TCA and milling experiment showed that lengthwise curvature modification could be achieved without changing cutter diameter, contact lines and transmission error could be controlled more easily. 