Single-point electromechanical (EM) cable subsurface mooring (EMC-SSM) is an innovative subsea structure used to measure several ocean parameters throughout a water column in real time. However, designing an EMC-SSM that can adapt to rough sea conditions is challenging owing to the relative fragility of EM cables. In this study, an optimisation design method based on parameter sensitivity analysis was proposed to improve the responses of EMC-SSM. The unreasonable design and optimisation value were determined by evaluating the slopes of the sensitivity curves of four response parameters: the minimum and maximum tensions, maximum declination angle, and maximum curvature to the net buoyancy of the main float and big sensor package (BSP); drag; added mass coefficient of the BSP; and net weight and hydrodynamic coefficient of small sensor packages. We found that the proposed optimisation design method is effective in identifying and optimising the unreasonable configurations of mooring similar to the EMC-SSM.