Parallel line gear pair (PLGP) can achieve pure rolling meshing along its theoretical contact curves under the ideal condition. However, the actual meshing positions may deviate from the theoretical ones under the actual operating conditions, which may result in the alternation of contact pressure distribution and cause relative sliding of the meshing surfaces. Contact pressure and relative sliding are two main factors causing surface wear. A calculation method of surface wear of plastic line gear (LG) pair under dry friction conditions was studied theoretically and experimentally, taking a polyoxymethylene (POM) PLGP as an example. Based on the geometric model of PLGP considering the misalignments under the actual operating conditions, this method employs the three-element model and influence coefficient method to compute the contact pressure with the consideration of the viscoelasticity of the gear material. Archard’s wear equation is applied to calculate the surface wear depth. Results of the wear experiment of the POM PLGP specimens validated the feasibility of the calculation method. This study provides a necessary basis for the engineering application of plastic LG pairs.