In this study, we used the 183 translation probabilities between RNA codons and mutated amino acids to construct the theoretical distributions of mutated amino acids in hemagglutinins of influenza A virus. We then compared the actual distributions of mutated amino acids from 953 hemagglutinins with their theoretical ones. The results demonstrated that mutated amino acids generally follow the direction of the theoretical distributions governed by RNA codons. This, in turn, highlights the mutation trend of amino acids in hemagglutinins and provides a method for estimating possible mutations in a protein according to its theoretical distributions of mutated amino acids.