In this paper,the factors for SSTA development in finite sea area of low latitudes are discussed with simpler linearized equations,According to the coupling conditions at the interface of atmosphere and ocean,we studied the horizontal scale relations of air-sea coupled system satisfying the dynamical and thermodynamical requirements,From the fluctuation viewpoint,the relation of horizontal scales and wave numbers are presented for a linear atmosphere-ocean coupled system with the same time scales.At last part of the paper,the state of fluctuations is examined for different sea scales under certain given atmospheric scale with a simpler two-layer coupled model,and some significant results obtained.