The invention relates to a preparation method of an oil absorbent, in particular to a method for preparing the oil absorbent from low grade diatomite. The preparation method comprises the following 9 steps of: low grade diatomite raw material selection, crushing, acidification treatment, washing, drying, adding of the assistant (3%-5%) Na2Cl (industrial salt) and mixing, roasting at 750-850DEG C, breaking winnowing, and adding of the silane coupling agent KH-550 (gamma-aminopropyl triethoxysilane, Nanjing Union silicon Chemical Co., LTD. 5%-7%) to conduct modification treatment, thus obtaining the particle and powder diatomite oil absorbent. The oil absorbent has simple and easy preparation method, wide raw material sources and low cost, solves the current solid waste pollution problem of low grade diatomite, opens up a new way for low grade diatomite application, and realizes recycling of waste materials. The oil absorbent has good selectivity, strong absorptivity up to 500% of its own weight, rapid oil absorption speed, small oil absorbent stacking density, a melting point of 1450DEG C, and a high use temperature, and can float on the water surface before and after oil absorption, thus being convenient for recovery and post-treatment.