Broken tooth roots due to mounting errors in helical gears at high speeds and heavy loads are a common problem in steel mills. Based on the contact characteristics of helical gears, this paper uses the finite element method to simulate the bending stress at the root of the pinion gear and the contact stress at the tooth surface under two cases of different center distance error and axis parallelism error, analysis of the reasonable fluctuation range of the maximum principal stress and the maximum contact stress is obtained by fitting the relationship between these two stresses and the variation of the two errors, and then the bending and contact strengths are calibrated based on the simulation results. The study shows that the gears’ strength can meet the requirements when installed according to the grade seven gear transmission accuracy of the national standard of involute cylindrical gear accuracy, under the case of center distance error; under the case of the parallelism error, the inclination angle should be controlled within 0°3′2′′ to meet the minimum service safety factor conditions. The steel mills strictly control the above-mentioned installation accuracy when using helical gear drives, which can avoid gear tooth root breakage and improve the efficiency and reliability of gear drives.