Researches on monitoring the body fluid changes have attracted much attention during recent years. Real-time bioimpedance deviation detection has good potential for evaluating body fluid changes. In this paper, an improved device based on the self-balancing half bridge and the 4-electrode technique is proposed, which is able to detect minor bioimpedance deviations. The 4-electrode technique is used to remove the interference from contact impedance. Furthermore, the automatic balancing half bridge is utilized to deduct the high-level static impedance baseline, and thus, the dynamic range is enlarged. Moreover, a digital lock-in algorithm based on oversampling improved the effective resolution of the system. Validation experiments show that the minimum bioimpedance deviation measured in the experiment can reach 1 Ω even when the bioimpedance baseline is 10 kΩ and the noise level is low. The system has advantages of high resolution, high magnification, large dynamic range, and good self-adaptability for bioimpedance deviation detection.