Airflow into the lung is the result of negative alveolar pressure caused by the simultaneous action of diaphragm contraction and rib cage expansion. However, all of the determinate CFD simulations on airflow and particle deposition in the human upper respiratory tract to date have employed a fixed boundary condition, i.e. uniform or axial symmetrical velocity profiles at the mouth inlet and outlet/opening boundary at the end of the airway. In this study, the realistic breathing mechanism, i.e. expansion of the pleural cavity, is mimicked by the fluid-structure interaction employing a junction box routine to assign the motion of the moving wall boundary at the bottom of the extra-thoracic airway model with an extra bell-mouthed tube. The airflow and particle deposition are investigated, particularly for the movable boundary cases, and compared with the fixed cases. In Part I, the flow characteristics which are important factors for particle deposition, i.e. pressure drop, turbulence intensity, secondary intensity and velocity profiles, are illustrated at three flow rates, as well as the detailed recirculation flow in the trachea region. Compared with the fixed boundary cases, several notable differences are found: (i) the velocity profiles at the mouth inlet are not uniform or axially symmetric, but askew with highspeed flow shifted to the upper wall of the mouth inlet tube, and a more evident recirculation zone occurs near the lower wall of the inlet tube; (ii) the pressure drop and turbulence intensity are lower and the secondary intensity is higher than that in the fixed boundary case because of the askew profile in the mouth cavity and modified recirculation flow in the trachea region; ( iii) at three flow rates, the secondary intensities exhibit identical characteristics before the larynx where the turbulence fluctuations are very weak, but discrepancies are evident in the trachea region because of the high level of turbulence intensity and different pattern of recirculation zones; (iv) the length of the recirculation zones increase at three flow rates and the minor separation and reattachment occur in the middle portion of the recirculation flow, while the recirculation flow layer is divided into two parts at higher flow rates; (v) the peaks of the secondary flow intensity in the trachea region are located near the flow separation and reattachment points (both the major and minor) and the peaks of turbulence intensity in the trachea seem to correspond to the major separation and reattachment region.