Calculation of the deflection and tension of the membrane elements in geosynthetic-reinforced and piled (GRP) embankments is a controversial issue yet to be resolved. However, little attention was paid to the evaluation of the strain distribution in geosynthetic reinforcements and the three-dimensional displacements of the geosynthetic materials. Physical model tests of membrane effect of geosynthetic reinforcement in GRP embankments with different pile spaces were carried out using a self-designed equipment. In the model tests, the geosynthetic reinforcement was loaded by applying air pressure, the strains in geosynthetic reinforcement were monitored in different locations. It is shown that: the strain and tension of geosynthetic reinforcements are unevenly distributed, and the three-dimensional deformation of the geosynthetic above square-distributed piles can be represented by a parabolic face and a parabolic cylinder; the applied loads are carried mainly by 4 tensile strips between two adjacent piles, and the calculated tension in geosynthetic reinforcements is much larger than the experimental results. Based on the modeling test results, a new calculation method for the tension in geosynthetic reinforcements is tentatively presented in this paper.