To avoid the unexpected effects of "undercutting," and "topping" occuring in the case of "small multi-thread reduction ratio" plane re-enveloping hourglass worm transmitting, an optimum method based on the meshing principle analyzing systhesisly the multiple parameters of the transmitting system is successfully applied. A six threads worm is employed and designed by the optimum method above, the gear-grinding test result shows that no "undercutting" and "topping" occur in the worm investigated which has demonstrated the accordance between the theoritical analysis and practical results.