Blast furnace slag (BFS) was added into drilling fluid as a mud cake modifier (MCM) for the reuse of BFS. The effects of MCM on drilling fluid rheology and shear strength of cement- formation interface (SSCFI) were evaluated. The experimental results showed that the optimized formula of the drilling fluid has 1.0 wt% MCM. The simulation results showed that SSCFI increased significantly with the addition of MCM. Based on the mechanism analysis of BFS as an MCM to improve SSCFI, it is found that the transformation of mud cake to agglomerated cake (MTA) can be the main reason. The trial result of horizontal well I306H in Laojunmiao oilfield in China showed that the one-time success rate of cementing operation was 100% and the high quality rate was 79.78%. This horizontal well is flowing well after perforation with oil production of 11.5t/d and the water cut is only 9%. Compared to the adjacent horizontal wells after fracturing, the oil production rate is increased by 228.57%-259.38%. Obviously, the effect of BFS as an MCM is self-evident. Meanwhile, this research provides a new way for the recycling of BFS.