This paper focuses on experiments and finite element modelling of flexible plug expansion joints (Asphaltic Plug Joints, APJ) subject to thermally induced horizontal movements. Five geometric and structural key parameters that influence (APJs) responses under thermal movements are studied: (1) joint length; (2) joint thickness; (3) joint width; (4) anti-bonding mat; (5) movement-aid spring. The viscoelastic computational finite element models are based on properties determined with a special cyclic coaxial shear test (CAST) and validated by an integrated approach incorporating cold temperature repeated movement capacity tests with a special Joint Movement Simulator (JMS) and a 3-Dimensional Digital Image Correlation system (3D DIC). It was found that the increase of joint width significantly reduces the stress at the interface between the mastic asphalt and APJ. The results also showed, that thin joints generate lower stress levels in APJ under thermal condition. Moreover, peak stresses in APJ appeared controlled mainly by the total size of the debonded region and the horizontal movement applied. The main findings are considered valuable for superior structural design, geometry selection and construction guidelines for APJ.