As performance requirements for ship systems increase, marine gear transmission devices are evolving towards ultra-high power, high speed, and larger sizes. Wide-faced helical gears in these systems often need to transmit large torques. The larger tooth width makes them very sensitive to shaft misalignment, which is difficult to predict and measure in transmission systems. An effective method to assess the contact state of the gear surface is through the tooth root stress (TRS) of the gear. To this end, we established a three-dimensional finite element model of a wide-faced helical gear pair to analyze the relationship between the contact state and the TRS under misalignment errors, and identified a parameter that represents the degree of uneven load and TRS distribution. Through the relationship between the misalignment angle and the degree of uneven stress distribution, the load distribution can be calculated from the TRS distribution. Furthermore, we investigated the effects of basic gear parameters—such as module, helix angle, and face width—on the misalignment characteristics of helical gears.