The mechanical response of asphalt layer of asphalt pavement is an essential parameter for pavement design. An accurate characterization of the mechanical response of the in-situ asphalt layer is of great significance to improve the reliability of pavement design. The strain data of the service flexible asphalt pavement were collected by field vehicular loading tests. The characteristics of strain waveforms and the strain values under various loading conditions including different axle loads, vehicular speeds and temperatures were investigated by the statistical approach. The results show that the strain waveforms at the bottom of the asphalt layer are significantly influenced by the loading position of the tire. The axle load, temperature and vehicular speed all have significant effects on the strains at the bottom of asphalt layer, but do not significantly change the strain waveform shapes. The increase in the axle load/temperature or the decrease in the vehicular speed all leads to an overall increase in the peak-to-peak strains values of transverse and longitudinal strain pulses. Compared with the waveforms at the bottom of the asphalt layer, the strain response waveforms at the bottom of the ATB layer show significantly viscous behaviors. The interaction of two factors, axle load and temperature, on the mechanical response of the pavement is obvious. The effect of axle load on the extreme values of horizontal-vertical strains at different temperatures was shown to be significantly different. The interaction between vehicle speed, axle load and temperature was not significant. In addition, the model for describing the relationships among strains, vehicular speed, axle load and temperature are established, to predict the strain values under various loading conditions. The research findings are helpful to evaluate the real stress state of flexible pavement asphalt layer, and provide reference data for the design of flexible pavement in China.