  PSR B1259-63 is a $\gamma$-ray binary system, where the compact object is a
pulsar. The system has an orbital period of 1236.7 days and shows peculiar
$\gamma$-ray flares (in 100\,MeV--300\,GeV) after its periastron time. We
analyzed the \textit{Fermi}-LAT observation of PSR B1259-63 during its latest
periastron passage, as well as its previous three periastrons. The bright GeV
flares started about 60 days after the periastron epoch in 2021. This delay is
larger than that around the 2017 periastron and much larger than earlier
periastrons. The delay of the GeV flux peak time in each periastron passage is
apparent in our results. We discussed the possible origin of this delay and
made a prediction of the GeV flux peak time in next periastron passage, based
on observation of the previous delays.
