Synchrotron radiography was performed during the solidification of Al-20wt.%Cu hypoeutectic alloy to study the competitive grain growth dynamics and a mathematical model consistent with the experimental results was established. The experimental results showed that for the diverging competitive growth, the “normal elimination” rule played a role. For converging competitive growth, the growth rate of the primary dendrite arms in the non-preferentially oriented grains was accelerated due to constitutional supercooling, resulting in the phenomenon of “abnormal elimination” in which the non-preferentially oriented dendrites would eliminate the preferentially oriented dendrites. The critical conditions between “normal elimination” and “abnormal elimination” of converging competitive growth were explored using a mathematical model. The mathematical model named the “secondary dendrite arm blocking model” among dendrite growth rate, dendrite arm spacing and dendrite inclination angle was established, which could quickly predict the orientation of grain boundaries and the type of elimination.