The structure of sound speed in ocean has a strong influence on underwater sound propagation. For underwater target detection and underwater acoustic communication, it is of great significance to obtain the temporal and spatial distribution of sound speed profile. Since the limitations and applicability of vertical grads method in structural analysis of sound speed in the Indian Ocean, optimal partition method is developed to calculate the sound spring layer. The study focuses on analysing the eigenvalues of sound speed profiles (SSPs) in the north-central Indian Ocean based on the last 10 year data of array for real-time geostrophic oceanography. The vertical structure characteristics of sound speed are investigated in the Indian Ocean, and the laws of temporal and spatial variation are obtained. The distribution of the fitting accuracy with the order of empirical orthogonal functions (EOF) are found. The physical mechanisms of the temporal and spatial variation of SSPs are revealed with the marine physical environment in Indian Ocean. The simulation results show that optimal partition method is suitable for the area to judge the structure of spring layer, and the parameters of the corresponding criterion is also proposed. The regional distributions of the fitting accuracy with the order of EOF are more obvious than with the seasonal variations. The deep channel axis exists at south 5S and there are three spring layers between 15S and 25S. The structures of SSPs in the Indian Ocean can be classified into four types: single spring layer, double spring layer type Ⅰ, double spring layer type Ⅱ and three spring layer, and for seasonal models: spring model, summer model, autumn model and winter model. The analysis results of the SSPs can provide some reference value for acoustic propagation and the sonar systems.