The novel toroidal worm-gearing is a kind of worm transmission with spherical meshing elements, which is made up of worm, steel balls and worm gear, and its loading capacity and adaptability to errors can be improved by the mismatched technology applied to the meshing pair. Based on the directrix of worm surface, the mathematic model of worm surface is established, and the directrix-based forming method for machining worm surface is proposed. Further, the principle error in the machining process is analyzed, and the theoretic and real tooth surfaces of worm are fitted and compared on OpenGL platform. The results show that the tooth profile error can be controlled at the range of 0~1×10-5mm, and it is always 0 at the pressure angle.