The two-body system dynamics of the aircraft-rocket during the built-in gravity air-launch process is studied. And the transport aircraft is taken as the platform. According to the mechanical conditions of the rocket and the degree of freedom of the relative movement， the rocket’s exit-movement is divided into five stages in sequence. The mechanical conditions for the transfer of the stages are provided. Based on the Newton-Euler method， the aircraft-rocket coupling dynamics models for the first four stages are established. It defines a variety of dangerous abnormal situations that may occur when the rocket exits the cabin， and the mechanical or geometric conditions under which the abnormal situation occurs are provided. Then a numerical simulation is carried out for the whole process. If the initial launch conditions are reasonable， the simulation will go through the first four or three normal stages in sequence until the rocket leaves the cabin. If the trigger conditions of the abnormal situation are met， the rocket will be dangerous when it exits the cabin， and the simulation is meaningless. So the simulation is aborted and the initial launch conditions need to be adjusted， and further simulation verification is required. The system dynamics model and simulation data obtained in this paper may provide references for the engineering design of parameters of the air-launched rocket， as well as the practical settings of the initial conditions of the air-launch.