The invention relates to an extending method of a horizontal well safe density window particularly. The extending method comprises the steps that 1 1.5% of bentonite powder, 1% of saw dust, 1% of blocking agent and 1% of mica powder are added into 100 squares of drilling fluid gradually in a borehole with a standard of 83/8''according to a mass-volume percent; 2 the displacement of a drilling pump is lowered when the drilling pump enters a leakage layer; 3 when the leakage layer opens, the rate of penetration is controlled, solid control is strengthened at the same time, and harmful solids in the drilling fluid are removed; 4 the well deviation in an oil shale section is controlled; 5, the orientation of the borehole is conducted with a roller bit. According to the extending method of the horizontal well safe density window, blocking agents of different granularity are added gradually through a mud system, the leak-proofing formula is improved, the concentration of the blocking agent when added into the well is guaranteed to be at 3%, blocking is strengthened, and the strength of a leaking well wall is improved; the displacement of the drilling pump is lowered, the expansion of a crack is prevented, and the collapse pressure of the balance oil shale section is lowered, so that the safe density window is extended.