A powder natural asphalt modified product for oil drilling liquid anti-sloughing agent and the preparation method are provided. The anti-sloughing agent comprises natural asphalt produced in Junggar zone in Xinjiang Karamay city and a modifier. The modifier is a non-ionic surfactant, an organic cationic polymer (polymeric quarternary ammonium salt) and non water-soluble solid accessories. The preparation method is that equally mixed method, kneading method or wet piling method are adopted; the production process is simple and reliable; waste materials and environment pollution does not exist. The softening point of the natural asphalt in the product is 130 DEG C. to 150 DEG C.; the lipophilic component content of the product is not less than 85 per cent; the water-soluble composition content is not more than 10 per cent. The invention has no toxin and has high asphalt content, high lipophilic component content, low water-soluble composition content and low effect of the rheological property of the drilling liquid, which is suitable for deep drilling as the anti-sloughing agent.