The invention provides a method for removing fine solid particles from medium oil in a fractionating tower in an ethylene unit. The method comprises the following steps of: performing mini-hydrocyclone separation on medium oil extracted from an ethylene cracking fractionating tower to remove fine solid particles entrained by the fractionating tower; concentrating coke powder entrained in the medium oil stepwise by performing multi-stage mini-hydrocyclone separation; performing hydrocyclone concentration or filtering separation on a solid particle enrichment phase; and recovering the medium oil and coke powder respectively to reduce the discharge amount of solid wastes. The invention further provides a device for removing fine solid particles from medium oil in the fractionating tower in the ethylene unit. According to the method and the device, the heat exchange efficiency of medium oil is increased, the erosion of coke powder entrained in the medium oil on a pipeline and equipment is reduced, continuous running period of the entire device is ensured, and the energy consumption is lowered.