The invention discloses a preselecting method of low-grade magnetic iron ore, comprising the steps of crushing the raw ore of the magnetic iron ore by coarse crushing, intermediate crushing and fine crushing operations, conveying to subject to ultra-fine crushing operation, and then crushing by a high-pressure roller mill, scattering the crushed product by a wet cylindrical sieve, conveying the ore on the cylindrical sieve to the vibrating sieve by a belt to perform wet gradation, conveying the ore on the vibrating sieve to a dry magnetic separator by the belt to discard the waste, returning the ore concentrate of the dry magnetic separator to the high-pressure roller mill to crush the ore concentrate again, perform magnetic separation and discarding tailings of the ore concentrate below the vibrating sieve and the ore concentrate below the cylindrical sieve in a wet coarse grain intermediate-field intensity magnetic separator, and selecting the coarse ore concentrate of the intermediate-field intensity magnetic separator by a grinding separation technology. The invention has the advantages that the invention selects quantities of qualified mine tailings by wet magnetic separation before grinding ore, reduces ore grinding work, reduces ore grinding energy consumption and has high system processing capacity; the invention can be widely used in magnetic ore dressing plant, and in particular suitable for treatment of preselecting low-grade magnetic iron ore with the iron grade lower than or equal to 22%.