Provided are an ultra-clean rare earth steel and an occluded foreign substance modification control method, the steel includes 10-200 ppm of rare earth elements, 50% or more occluded foreign substances in the steel are dispersed into RE-oxygen-sulfide with the average equivalent diameter Dmean ranging from 1-5µm in a spherical shape or a substantially spherical shape or a granular shape; according to the method, at least 80%, preferably at least 90%, of Al 2 O 3 occluded foreign substances in the steel are modified into RE-oxygen-sulfide, compared with steel with the same components without rare earth, the total amount of the occluded foreign substances in the steel is reduced by 18% or higher, the cracking probability caused by occluded foreign substances such as Al 2 O 3 in traditional pure steel is reduced, the mechanical performance such as the fatigue life of the steel is remarkably improved, precise control over the modification type, distribution and dimension of the occluded foreign substances in the steel is achieved, the method is suitable for researching and producing of high-performance steel in more varieties.