The polarization frequency of free radical solution in Overhauser geomagnetic sensor determines the quality of the Larmor precession signal generated by the sensor. To obtain the polarization frequency accurately, a test apparatus was designed in this paper, which can overcome existing problems in the presently used apparatuses, such as lower resolution, complex operation, etc. The proposed apparatus adopts a high-resolution direct digital synthesis as the controllable radio frequency (RF) signal source. Meanwhile, an analog-to-digital converter synchronization acquisition technology combined with a normalization approach is proposed, which effectively solves the problem of the uneven amplitude-frequency characteristic in the range of 50 MHz-100 MHz. Moreover, the apparatus is integrated by adopting the RF power and applying the weak signal amplifier as an auxiliary measurement channel. The equivalent circuit of the sensors resonant cavity was simulated, and the efficiency curve of the adjustable capacitors to the resonant frequency and the quality factor were obtained. The simulated results were further verified by testing the resonance cavity characteristics of a commercial Overhauser geomagnetic sensor under different conditions. Furthermore, the relationship between the polarization degree of the free radical solution and RF excitation power and time were also obtained. The testing methods and results were given, and the experimental data were analyzed. Finally, the experimental results demonstrate that the proposed apparatus can measure the polarization frequency of the free radical solution, the bandwidth, and the quality factor, accurately. Furthermore, it can be used for the determination of the polarization power and time during the design process for an Overhauser magnetometer.