An avian-origin human-infecting influenza (H7N9) virus has recently been identified in China. Here, we have evaluated the viral hemagglutinin (HA) receptor binding properties from two human H7N9 isolates, A/Shanghai/1/2013 (SH-H7N9) (containing the avian-signature Q226) and A/Anhui/1/2013 (AH-H7N9) (containing the mammalian-signature L226). We found that SH-H7N9 HA preferentially binds the avian receptor analog, whereas the AH-H7N9 HA binds both avian and human receptor analogs. Furthermore, an AH-H7N9 mutant HA (L226Q) has dual receptor binding property, indicating that other amino acid substitutions contribute to the receptor binding switch. The structures of SH-H7N9 HA, AH-H7N9 HA, and its mutant in complex with either avian or human receptor analogs show how the AH-H7N9 can bind human receptors, yet also retain the avian receptor binding property.