The invention discloses a preparation method for a magnetic carrier in a processing technique of a sewage biological fluidized bed, which comprises the following steps: (1) magnetic powder is added into a reaction kettle; in the stirring speed of 40 to 100 r/min, inorganic acid and organic acid are sequentially added and heated till the temperature is risen to 65 to 80 DEG C and the reaction lasts for 1.5 to 2 hours; (2) reactant is transferred to a dying cabinet, dried for 0.5 to 1 hours in the temperature of 80 to 100 DEG C, anhydrated, cooled and then crushed by a grinding crusher and sieved with a mesh screen with 100 meshes to obtain granular magnetic carriers. The preparation method has simple process route, low energy consumption, high efficiency and low cost. The magnetic carrier obtained by the preparation method is a shell structure; microorganisms easily grow biological membranes on the magnetic carrier; residual biological membranes can be removed by a magnetic drum separator, thus avoiding the huge loss of organisms. The biological fluidized bed using the magnetic carrier is suitable for fields such as standard reaching discharging treatment of town sewage, synchronous denitrification and dephosphorization of high nitrogen and phosphorus medical waste water, nitrification and denitrification of garbage percolate, coking wastewater and the like.