Abstract: Based on the comparison of the full displacement mode and conventional displacement mode of Galerkin equations, the exact conditions for the establishment of Galerkin equations is derived. The association of a displacement mode and the bubble function is demonstrated. The combination program of a liner element and a bubble function is proposed. Using the condition of precise establishment, by the analysis order of magnitude, retaining the main impact item of a bubble function, the analytical expression solving for bubble functions is avoided. The derivative accuracy has reached the order of h4, compared with the liner element that the convergence precision is the order of h2. The calculation does not be essentially increased. Theoretical analyses and numerical calculations show that this unit is a superior performance than a high-dimensional element.