Simplified rails and armature as straight conductor with line current, the model of electromagnetic launcher was established. The magnetic intensity of the magnetic probe coil center position was calculated by using Biot-Savart law and the principle of induction electromotive force. Under the assumption that the magnetic field is a uniform magnetic field, the electric potential of the coils was elaborated. An applied probe was designed and the model was verified by comparing with the test data. The issue that probe’s distance change and angle deflection under vibration was raised, the fault states in three groups single shot were measured and analyzed. Simulation and experimental data show that the magnetic probe do not cause failure with a small range of distance variation, while the angle deflection has an interferer on fitting velocity, with the deflection adding the fitting velocity falling.