As one of the important circuits of the rubidium atomic clock, the C-field circuit plays a significant role in the rubidium atomic clock’s performance. Generally, a C-field circuit is used to provide a constant current to a C-field coil by forming a weak magnetic field to select two hyperfine mF = 0 energy levels and thus fine-tune the transition frequency of 87Rb. In this way, the C-field circuit has a direct influence on the frequency stability of the rubidium atomic clock. In this paper, by studying the principles and functions of the C-field in a rubidium atomic clock, we propose a modified C-field circuit, which not only provides constant current to the C-field coil but also provides a pathway to improve the performance of the rubidium atomic clock significantly by optimizing the start-up characteristics and temperature coefficient carefully. In addition, according to the theoretical analysis and test verification, the rubidium atomic clock with the proposed C-field circuit improves the temperature coefficient from −1.2 × 10−13 to 2.7 × 10−15/°C, and in addition, the stability of long-term frequency can reach 3.88 × 10−15 at 105 s.