Based on the analytical trial function method,a novel 5-node plane quadrilateral membrane element is developed following the unsymmetric formulation concept,in which two different test and trial functions are simultaneously used.The test function employs the isoparametric coordinate interpolation to satisfy inter-element compatibility,while the trial function adopts the analytical solution of displacements in Cartesian coordinates to satisfy the completeness requirements in physical space.The resulting element US-ATFQ5 is insensitive to various serious mesh distortions,and can satisfy both the classical first-order patch test and the second-order patch test for pure bending.