In some recent publications, a gear mechanism named Space-Curve Meshing-Wheel (SCMW) has been proposed by the present authors. This paper presents the geometry of the Helix-Curve Meshing-Wheel (HCMW), the most common SCMW at present. The driven contact curve equation is optimized by removing a redundant length parameter. Then, both geometric constraints and interference-proof conditions are calculated in the following three aspects: contact curves, HCMW pair and HCMW train. A parameter selection of a novel reducer with HCMW trains is shown as a practical example. The theories in this paper are mainly applied to determine the feasibility range of the geometric parameters within the HCMW, and provide foundation to its industrial standardized production.