Abstract: The principle of a new type spiroid worm-gear drive was discussed in the paper.In the new transmission way,the engaging-in process of the gear surface is based on the principle of external meshing crossed helical gear drive.And the engaging-out process of the gear surface is based on the principle of internal meshing crossed helical gear drive.All the profiles of the teeth are involute,instantaneous contact state is a beeline on the surfaces,and the surfaces of the teeth are helical involute surface whether on the gear or on the worm.The structural characteristics of this worm drive were analyzed.For the double lead spiroid worm-gear drive,the principia to determine the kinematic and geometric parameters was presented,Installation conditions for correct transmission were discussed.And a calculation example was provided.