Helical gear with asymmetric involutes is a new kind gear which driving side surface is large pressure angle profile. The numbers of meshing tooth are discussed during helical gear pair meshing process, thus the length of total contact lines is determined easily. It is difficult to calculate contact stress by existing formula because this kind gear is non-standard. A calculating formula is derived which can calculate contact stress of helical gear with asymmetric involutes in this paper. A computer’s software is written in order to calculate instantaneous contact stress of contact line. Contact stress is compared between symmetric helical gear and asymmetric helical gear.