As a common power transmission component in the heavy industry field, reducers are widely used in wind turbines, lifting mechanisms, rail travel mechanisms, and many other occasions. If the transmission gear fails during operation, it will lead to serious consequences. Therefore, it is necessary to conduct reliability analysis on them to reduce the number of failures. This paper presents an analysis of the operating environment for a specific level of gears in a gearbox, taking into account potential gear failures and the impact of failures in other components. The reliability of the gear’s operating environment is modeled using the fault tree analysis method. By collecting relevant event failure rate data and failure time data for seal failures in gearboxes obtained from the company, parameter estimation of the failure distribution for seal failures is performed using the maximum likelihood estimation method. The data obtained from the calculation of the distribution functions for each event failure is used to conduct quantitative analysis of the fault tree. Based on the results of the quantitative analysis of the fault tree, the weak links in the system, which are important failure factors affecting the normal operation of the gear, are identified. Guiding improvement suggestions are provided to enhance the reliability of gearbox gear operations.