Hydrate formation and decomposition are major challenges in marine hydrate development drilling. Filtrate reducers are important additives in water-based drilling fluids that reduce methane hydrate formation. However, research on the influence of commonly used filtrate reducers in drilling fluids on hydrate formation is limited. Here, we investigated the effects of three filtrate reducers, humic acid potassium (KHm), hydroxypropyl starch (HPS), and hydrolyzed polyacrylonitrile sodium salt (Na-HPAN), on methane hydrate formation kinetics and those of different concentrations under an initial pressure of 6 MPa and different initial pressures (6, 8, 10, and 12 MPa) in the same concentration system. The KHm solution (1–3%) promoted hydrate formation at 4 °C and 6 MPa, the Na-HPAN solution (0.3–1.5%) inhibited hydrate formation, and the HPS solution (0.1–1.5%) inhibited hydrate formation in the first 10 h of reaction but promoted a large amount of hydrate formation after the inhibition failure. At 4 °C, higher initial pressures led to earlier and faster formation of CH4 hydrate in large quantities in the filtrate reducer solution. An extreme gas hydrate conversion value was observed, indicating that the initial pressure is crucial when using a filtrate reducer. These observations provide a crucial basis for flow assurance and water-based drilling fluid design.