The invention discloses a meshed cylindrical gear pair in a linear-involute tooth profile, which comprises an internal gear and a gear in meshing transmission with the internal gear. The tooth profile of the internal gear is linear, and the tooth profile of the gear in meshing transmission with the internal gear is involute. The tooth profile of the internal gear is changed into a straight line from an involute, grinding teeth can be shaped only by linearly finishing by a grinding wheel, the finishing efficiency is remarkably improved, and the precision can reach grade 4 to 6; and the internal gear adopts low-carbon steel carburization quenching, not only has high tooth surface strength, but also does not need to adjust quality before quenching, and improves the gear shaping processing efficiency greatly, so the meshed cylindrical gear pair radically solves the problems of low processing efficiency of the internal gear, poor precision and low tooth surface hardness, and promotes overall performances of a high-speed heavy duty speed reducer (speed increaser) containing the internal gear pair made in China.