The invention discloses a preparation method of a high-hardness and high-strength expanding pyramid of a mechanical expanding mill. The preparation steps are as follows: (1) smelting, spheroidizing, inoculating and casting; (2) annealing; (3) plugging holes with refractory clay and asbestos, blocking grooves, sealing center holes so that the preparation works are completed; (4) charging the expanding pyramid of which the large end downwards faces into a tooling of which the bottom is a plane, placing the tooling inside a well-type heating furnace and drying the refractory clay so as to complete the charging operation; (5) by controlling the temperature-raising speed and the temperature-raising temperature, heating; (6) by controlling the temperature-raising speed, the temperature-raising temperature and the temperature-raising time, austenitizing; (7) by controlling the time and temperature, carrying out three-stage quenching; and (8) by controlling the time and temperature, tempering. The high-hardness and high-strength austenitic-bainitic nodular cast iron expanding pyramid subjected to above treatment steps has no defects such as quenching cracks, high quenching depth, good compressive strength, impact toughness and hardness, can meet the needs of machining and cannot be deformed after machining. The expanding pyramid is suitable for preparing steel tubes having steel grade of X60-X80 and wall thickness of 10-18 mm.