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After in quiescence for 49 years, 4U~1730--22 became active and had twooutbursts in 2021 \& 2022, the onset and tail of the outbursts were observed byNICER, which give us a peerless opportunity to study the state transition andits underlying mechanism. In this work, we take both the NS surface andaccretion disk emission as the seed photons of the Comptonization and derivetheir spectral evolution in a bolometric luminosity range of 1\%--15\%$L_{\rmEdd}$. In the high/soft state, the inferred inner disk radius and the NS radiusare consistent well, which implies that the accretion disk is close to the NSsurface. For the decay stage, we report a steep change of the accretion diskemission within one day, i.e., the soft-to-hard transition, which could be dueto the propeller effect and the corresponding neutron star surface magneticfield is 1.8--2.2$\times10^{8}$ G. Moreover, the inner disk radius is truncatedat the corotation radius, which is similar to the propeller effect detectedfrom 4U~1608--52. The absence of the propeller effect in the hard-to-soft statetransition implies that the transition between the magnetospheric accretion andthe disk accretion is not the sole cause of the state transitions.

Return of 4U~1730--22 after 49 years silence: the spectral properties of  the 2021/2022 outbursts observed by NICER and Insight-HXMT and the  soft-to-hard state transition caused by the propeller effect