The vertical strain of an asphalt layer is essentially important when identifying rutting mechanisms or causes in asphalt pavement with a semi-rigid base course, which is a typical pavement structure in China. As a result, three directional strain measurements (longitudinal, transverse, and vertical strain) in the field, together with a pavement rutting survey and finite element simulation, were carried out in order to clarify the causes of rutting happening in Beijing heavy-duty expressways. First, the basic principal of a fiber Bragg grating sensor is explained, followed by the introduction of sensor installation in the field. Then, a three-dimensional finite element pavement model subjected to a non-uniform moving load was established to validate the dynamic response measurement on site. Finally, based on a comprehensive analysis including a pavement condition survey, numerical simulation, and strain measurement, the rutting mechanism of heavy-duty asphalt pavement in Beijing is discussed. The results show that the occurrence of shear deformation within asphalt layers results in the rutting development, in which material weakness, modulus mismatch of asphalt layers, and heavy-duty characteristics play important roles. Optimizing asphalt layers’ moduli, i.e., making them match, might make pavement bear the applied loading with integrity, thus averting the occurrence of excessive local strain, i.e., rutting.