H9N2 subtype avian influenza virus (AIV), which is mild but widespread, has been a concern due to the potential threat to poultry farming and public safety. AIV-infected poultry exhibit a complex range of various types of disease, which, coupled with the fact that AIV viruses are prone to genetic mutation and remodeling and exhibit a very high degree of variability, further increases the difficulty of immunization and diagnosis of avian influenza. We focused on the PB1 protein, a vital part of the virus’s ability to replicate. In this study, a truncated PB1 protein was designed and expressed in the prokaryotic expression system. Four hybridoma cells secreting the antibody specific to PB1 protein were screened using mouse immunization and cell fusion techniques. Two B cell antigenic determinants were identified by gradually truncating protein expression, and were conserved across different flu strains and located on the surface of PB1 protein. These findings could help make better vaccines against the H9N2 virus and might be important for controlling the spread of bird flu and protecting both animals and humans.