  Type I X-ray bursts in the ultracompact X-ray binary 4U 1820$-$30 are powered
by the unstable thermonuclear burning of hydrogen-deficient material. We report
the detection of 15 type I X-ray bursts from 4U 1820$-$30 observed by NICER in
between 2017 and 2023. All these bursts occurred in the low state for the
persistent flux in the range of $2.5-8\times10^{-9}~{\rm erg~s^{-1}~cm^{-2}}$
in 0.1$-$250 keV. The burst spectra during the tail can be well explained by
blackbody model. However, for the first $\sim$5 s after the burst onset, the
time-resolved spectra showed strong deviations from the blackbody model. The
significant improvement of the fit can be obtained by taking into account of
the enhanced persistent emission due to the Poynting-Robterson drag, the extra
emission modelled by another blackbody component or by the reflection from the
surrounding accretion disk. The reflection model provides a self-consistent and
physically motivated explanation. We find that the accretion disk density
changed with 0.5 s delay as response to the burst radiation, which indicates
the distortion of the accretion disk during X-ray bursts. From the
time-resolved spectroscopy, all bursts showed the characteristic of
photospheric radius expansion (PRE). We find one superexpansion burst with the
extreme photospheric radius $r_{\rm ph}>10^3$ km and blackbody temperature of
$\sim 0.2$ keV, thirteen strong PRE bursts for $r_{\rm ph}>10^2$ km, and one
moderate PRE burst for $r_{\rm ph}\sim55$ km.
