The time domain responses of the immersed tunnel element under typical load combination case during its immersion are investigated based on the 3D potential method. The varied cross-section complicated model is built. The typical environmental and immersion parameters are considered. The results of different line models are compared and investigated. Numerical results demonstrate the effects of the mooring lines’ condition. The “optimum” balance between relative large mooring stiffness and relative safety tension can be realized by combining the coarse calculation and hydraulic exact analysis.