To enhance the probe beam current of scanning electron microscopes and related devices， a thermal field electron gun immersed in a magnetic lens field was designed to collect electrons emitted from the cathode and increase the effective emitted beam current. The optical properties of the magnetic field-immersed thermal field electron gun were theoretically analyzed， and the performance of three magnetic lens configurations—single polepiece， double polepiece with axial gap， and double polepiece with radial gap—was simulated and compared. A magnetic field-immersed electron gun was subsequently developed based on the double polepiece with axial gap lens， and the variations in beam spot size， beam current， and angular current density in response to the magnetic immersion lens were examined. Experimental measurements of the magnetic field distribution of the immersion lens and the effective emitted beam current of the magnetic field-immersed electron gun were conducted using a dedicated platform. Results demonstrate that the measured magnetic field distribution closely matches simulation predictions. The effective emitted beam current reached 44.97 nA at a low acceleration voltage of 3.3 kV and 638.12 nA at a high acceleration voltage of 23.5 kV， representing an approximate fourfold increase compared with conventional thermal field electron guns.