Ensuring the sealing integrity of cement sheaths used for shale gas wells during hydraulic fracturing processes has become a major challenge. The sustained casing pressure, strong temperature difference stress, and cyclic loading and unloading affect the safe long-term production of shale gas wells. In this paper, we propose a new hydraulic sealing capacity (HSC) evaluation system for cementing interfaces. Specifically, this study aimed at understanding the influence law and failure mechanism of the interface sealing of shale gas wells before hydraulic fracturing. The results showed that the HSC improved with an increase in the flushing time and velocity. The HSC of the cement-formation interface was likely to be compromised if the cement slurry was contaminated with a spacer and oil-based mud (OBM). When the cement slurry was mixed with more than 10% OBM, channels formed easily in the cement sheath body. Moreover, the preflush injection sequence of “flushing fluid + spacer + flushing fluid” significantly improved the HSC of the cement-formation interface. Based on the experimental results and field operation experience, we determined that the optimal flushing time and flushing displacement were 15–20 minutes and 1.8 m3/min, respectively. Finally, the microstructure and distribution characteristics of the cement-formation interface under different conditions were obtained by computed tomography (CT) analysis, which explains the formation and failure mechanism of the cement-formation interface HSC. The development of harmful pores (R > 1 mm) was the main factor affecting the HSC of the cement-formation interface. The number of harmful pores in cement-formation interface increased significantly after being polluted by OBM. Combined with the field operation experience, the prefluid injection sequence of “6 m3 flushing fluid + 25 m3 spacer fluid + 6 m3 fresh water” is adopted, and the displacement volume is 1.4–1.6 m3/min, and the cement slurry is injected 10–15 m3 more, which can greatly improve the HSC of the cement-formation interface. Our findings highlight the need for more explicit considerations regarding the impact of cementing technology and actual operation on the HSC of an interface under the requirements of efficient production and extended well life.