Abstract: A state-of-the-art algorithm of the Particle Image Velocimetry (PIV)was proposed to accurately determine the flow field near arbitrarily moving and deforming boundary,which is usually associated with strong nonlinear coupling of fluid-structure interaction.This smart algo-rithm was employed to faithfully track the fluid-structure interface with large deformation;sub-sequently,a body-fitted arrangement of interrogation windows was made and the standard cross-correlation PIV algorithm was applied to get reliable flow field information near the boundary, providing a solid basis for fluid-structure interaction mechanism analysis and validation of numeri-cal calculations.The proposed algorithm went through bench-mark tests by using synthetically generated image pairs,showing the great potential for measuring the fluid flow around arbitrarily moving and deforming interface.Finally,the fluid flow of fluid-structure interaction associated with a flexible membrane,which was placed behind a square cylinder,was measured by PIV;the instantaneous flow fields near the violently moving and deforming membrane were determined.