In this work, remarkable transverse diffusion of minority carriers in the GaxIn1-xP top subcell of a GaxIn1-xP/GaAs double-junction tandem solar cell is revealed by the electroluminescence (EL) image surveying. As the forward bias is increased, the overall EL intensity rapidly increases, but the topographical distribution of lateral intensity becomes more uneven. By analyzing the relation between the measured EL emission intensity and diffusion parameters of electrically injected minority carriers, the transverse diffusion length of the minority carriers is determined to be ~93μm at the forward bias of 2.75V, which is 30 times larger than that of unbiased GaxIn1-xP single layer. Possible influence of such super diffusion of charge carriers on the conversion efficiency of tandem solar cells is discussed.