The invention relates to a preparation method of spherical nanometer yttrium oxide powder for an ultraviolet lamp tube. The method comprises the following steps that 1, after nanoscale sheet-shaped rhombus powder yttrium oxide and deionized water are mixed and quickly stirred for 30-60 minutes, a first mixed solution is prepared, wherein the mass ratio of nanoscale sheet-shaped rhombus powder yttrium oxide to the deionized water is (1-11):(92-101); 2, after ethanolamine and ethyl acetoacetate are mixed and stirred for 5-15 minutes at the constant speed, a first dropping solution is prepared, wherein the volume ratio of ethanolamine to ethyl acetoacetate is 1:(0.95-1.15); 3, after boric acid and glacial acetic acid are mixed and stirred for 15-30 minutes at the constant speed, a second dropping solution is prepared, wherein the mass ratio of boric acid to glacial acetic acid is 1:(0.95-1.15). The preparation method has the advantages of high radiation flux and low luminance decrease.