The magnetoplasmadynamic thruster is a typical representative of the high-power electric propulsion device, and the magnetoplasmadynamics process is its core operating mechanism. In order to understand the influence of applied magnetic field on its operating characteristics, the particle-in-cell particle simulation method combined with the scale model based on the self-similarity criterion is used to simulate the operating process of magnetoplasmadynamic thruster with applied magnetic field. The reliability of the model and method are verified by comparing with the experimental results. The plasma characteristic parameter distribution of the thruster during ignition is analyzed, and the influence of external magnetic field and cathode current on the thruster performance are discussed. The research results show that the construction of the discharge arc between the cathode and anode is a key step for thruster ignition and efficient operation. A low-intensity magnetic field is not conducive to the construction of a stable discharge arc, while the plasma beam is concentrated near the axis and the main thrust generation mechanism is the self-field acceleration. The discharge arc between cathode and anode is stable by applying a high magnetic field, and the main mechanism of thrust generation is vortex acceleration, which causes the thrust and specific impulse to increase linearly with the strength of the external magnetic field. The efficiency of the thruster increases with cathode current and the applied magnetic field intensity increasing. The discharge voltage increases with the augment of cathode current, but first decreases and then increases with applied magnetic field intensity increasing.