  It has been reported that there are two populations of Be/X-ray pulsars, with
the pulse period distribution peaked at $\sim$ 10 s and $\sim$ 200 s,
respectively. A possible explanation of this bimodal distribution is related to
different accretion modes in Be/X-ray binaries. In this work, we investigate
the spin evolution of Be/X-ray pulsars based on the magnetically threaded
accretion disk model. Compared with previous works, we take into account
several distinct and important factors of Be/X-ray binaries, including the
transient accretion behavior and possible change of the accretion disk
structure during quiescence. We demonstrate that current Be/X-ray pulsars are
close to the spin equilibrium determined by the balance of spin-up during
outbursts and spin-down during quiescence, and that the observed bimodal
distribution can be well reproduced by the equilibrium spin periods with
reasonable input parameters.
