Abstract: Flexible structures often experience large deformation in fluid with a significant fluid-structure interaction (FSI) effect. The FSI analysis, however, often has poor numerical stability and low convergence efficiency due to the locally drastic change of the physical fields induced by the computation errors at the fluid-structure interface. This paper proposes a gradient-piecewise-smoothed method by introducing weight coefficients into the Gradient-Smoothed Theory to improve the numerical stability and convergence efficiency in the FSI analysis. The physical field at the fluid-structure interface is linearly expanded in the smoothing domain with Taylor series, in which the gradient term is piecewise smoothed with the weight coefficients introduced, and a new gradient-piecewise-smoothed method is then proposed to mitigate the numerical perturbation and improve the performance in the FSI analysis. The proposed method is validated by comparing its numerical solutions with the experimental results in the literature, and the numerical performance is then investigated by analyzing a thin-walled plate in the viscous flow for different turbulence intensities and turbulence models. The results show that the proposed method is valid and accurate with notable improvement in numerical stability and convergence efficiency of the FSI analysis for different turbulence characteristics.