  After in quiescence for 49 years, 4U~1730--22 became active and had two
outbursts in 2021 \& 2022, the onset and tail of the outbursts were observed by
NICER, which give us a peerless opportunity to study the state transition and
its underlying mechanism. In this work, we take both the NS surface and
accretion disk emission as the seed photons of the Comptonization and derive
their spectral evolution in a bolometric luminosity range of 1\%--15\%$L_{\rm
Edd}$. In the high/soft state, the inferred inner disk radius and the NS radius
are consistent well, which implies that the accretion disk is close to the NS
surface. For the decay stage, we report a steep change of the accretion disk
emission within one day, i.e., the soft-to-hard transition, which could be due
to the propeller effect and the corresponding neutron star surface magnetic
field is 1.8--2.2$\times10^{8}$ G. Moreover, the inner disk radius is truncated
at the corotation radius, which is similar to the propeller effect detected
from 4U~1608--52. The absence of the propeller effect in the hard-to-soft state
transition implies that the transition between the magnetospheric accretion and
the disk accretion is not the sole cause of the state transitions.
