Magnetoplasma rocket engine has a broad application prospect in the deep space exploration, manned space flight and other space missions. The ion energy is converted into the directed velocity in the magnetic nozzle of the engine. The investigation into the detachment process of the plasma with the magnetic field is of great significance for improving the engine propulsion efficiency. However, there are roughly five kinds of physical mechanisms which can all contribute to the detachment process and make the detachment in the magnetic nozzle quite complicated. Furthermore, the ion temperature is much higher than the electron temperature in the magnetic nozzle of the magnetoplasma rocket engine due to the heating effect of the ion cyclotron resonance stage. As a result, previous numerical model which were based on the assumption of cold ions are unapplicable for the simulation of the engine. In this work, a fluid simulation model is developed which is used for simulating the magnetic nozzle in the magnetoplasma rocket engine. The model includes the electron and the ion of single charge. For the characteristics of the magnetoplasma rocket engine, the ion energy equation is added into the governing equations. In order to analyze the effect of the inertial detachment, the static electric field due to the charge separation is also included. The simulations are performed under the conditions of different inlet ion temperatures and background magnetic fields. The results show that the ion axial velocity gradually increases in the magnetic nozzle and the ion stream lines detach from the magnetic field lines gradually. The loss of adiabaticity is the dominant mechanism in the detachment process. The ion axial velocity increases with the inlet ion temperature rising, and the ion streamlines detach earlier from the magnetic field lines. The resistive diffusion is unaffected by the inlet ion temperature while the detachment interfaces of other three mechanisms all move toward the upstream. With the increase of the background magnetic field, ion axial velocity decreases and the angle included between the streamline and the axis becomes smaller. The loss of adiabaticity is still the dominant physical mechanism when the magnetic field is changed.