The invention belongs to the technical field of catalyst preparation, and particularly relates to a pre-conversion catalyst and a preparation method. The preparation method of the catalyst comprises the following steps: roasting magnesite fine powder, adding the roasted magnesite fine powder into a hot nitric acid solution, performing standing overnight, performing filtering to separate out filterresidues and filtrate, adding an active component and a nitrate solution of other assistants into the filtrate to obtain a mixed salt solution, adding the mixed salt solution into a constant-temperature storage tank 1, and adding a heated Na2CO3 solution into a constant-temperature storage tank 2; adding the reaction solutions in the constant-temperature storage tanks 1 and 2 into a precipitationreaction kettle at the same time, and controlling the relative flow rate to keep the pH value in the reactor at 8.0-9.5; and performing aging, washing, drying, roasting, and screening on the precipitate and adding an adhesive, and performing molding to obtain a finished product. The saturated sulfur capacity of the catalyst can reach 5%, and the service life is long; magnesite is adopted as a rawmaterial; the preparation method is simple, the cost is remarkably reduced, and large-scale industrial application is facilitated; the high hydrocarbon conversion capability is strong, and the carbondeposition resistance is strong.