Abstract: The rigid motion hypothesis in particle image velocimetry (PIV) is studied for the overlap problem of perspective particle image.The structure constraint based on three points is presented with mathematic expressions.The structure constraint is based on nearest matching assumption and machine vision theory.We have defined the motion feature plane in the paper.And the motion feature planes with the same movement pass the same line,which can be used to detect particle movements.At last,standard particle images were tested.The experimental results demonstrate the effectiveness of the proposed structure constraint.