Objective. Magnetic Resonance guided Radiation Therapy (MRgRT) integrates magnetic resonance imaging with radiation delivery, providing superior soft-tissue contrast and real-time treatment guidance. For preclinical MRgRT irradiators, a critical concern is whether the ambient magnetic fields compromise the radiation dose distribution. This study aims to investigate the influence of magnetic fields on X-ray tube electron trajectories and dosimetric outcomes. Approach. The system integrates a 0.5 T MRI scanner and a 225 kVp X-ray tube. The local magnetic field distribution was characterized via finite element analysis. Electron trajectories were simulated across various magnetic field strengths and alignment angles to quantify the X-ray focal-spot displacement. Monte Carlo simulation was then performed to evaluate the dose distribution within homogeneous and heterogeneous tissue-equivalent phantoms. Main results. In the proposed system, the fringe magnetic field across the X-ray tube electrodes measures 40.4 G and aligns nearly parallel to the electric field. This configuration restricts the X-ray focal-spot displacement to 0.15 mm, resulting in negligible dosimetric deviations. There is good agreement between the dose distributions in the phantoms with and without magnetic fields, maintaining gamma passing rates at 0.5 mm / 1% criteria even under a main magnetic field of up to 10 T. Significance. This study confirms that, for the small animal MRgRT system, main magnetic field as low as 0.5 T exerts no significant dosimetric impact, provided the magnet and the X-ray tube are well separated, and it may be unnecessary to perform dosimetric corrections.