The invention discloses a truncated-cone-shaped height-adjustable guardrail fixing cap and an application method of the guardrail fixing cap and relates to the field of construction of road traffic safety facilities. The guardrail fixing cap is mainly composed of a truncated-cone-shaped stand column, a cross-shaped cover plate, a guardrail beam hoop, an open wedge-shaped slide resistance ring, a guardrail beam and the like. The truncated-cone-shaped stand column is additionally provided with a fixing cap body, a fixing cap cover, an open-type fixing cap hoop, a fixing cap slide resistance ring, a fixing plate, a threaded rod, a lower nut, a lower middle nut, an upper middle nut, an upper nut and other structures. A plurality of fixing cap openings, fixing cap wing plates, wing plate hooks and stop pieces are further arranged on the outer edge of the bottom end of the fixing cap body. By combining the additionally arranged structures, stepless regulation of the height of the guardrail beam can be better improved, and stress safety for bearing a car load during collision can be better improved; the improved fixing cap further has the advantages of being simple in structure, convenient to construct, reliable in adjustment and the like, and therefore safe use of the traffic facilities is ensured.