The invention discloses a method for determining the content of solid-solution rear earth in steel. The method comprises the following steps: taking a steel sample with the mass of m1 as an anode, taking stainless steel as a cathode, and taking an anhydrous methanol solution of anhydrous lithium chloride and acetylacetone as electrolyte for electrolysis, weighing the steel sample after electrolysis to obtain the mass m2; drying organic matters in the electrolyte after electrolysis by evaporation, and adding nitric acid to decompose residues and obtain a test solution to be measured; taking cesium as an internal standard element, determining the mass m of solid-solution rear earth in the test solution to be measured and the mass m0 of solid-solution rear earth in a blank solution by an inductively coupled plasma mass spectrometer; obtaining the content of solid-solution rear earth in steel by W%=10<-6>*(m-m0)/(m1-m2)*100. The method for determining the content of solid-solution rear earth in steel has high accuracy and precision.