  We propose the thick-disc model of Gu et al. 2016 to interpret the transition
between soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) state
in NGC 247. As accretion rate increases, the inner disc will puff up and act as
shield to block the innermost X-ray emission regions and absorb both soft and
hard X-ray photons. The absorbed X-ray emission will be re-radiated as a much
softer blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hard
X-ray band and transits from the SUL state to the SSUL state. The $\sim 200$s
transition timescale can be explained by the viscous timescale. According to
our model, the inner disc in the super-soft state is thicker and has smaller
viscous timescale than in the soft state. X-ray flux variability, which is
assumed to be driven by accretion rate fluctuations, might be viscous
time-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable.
The bolometric luminosity is saturated in the thick disc; the observed
radius-temperature relation can therefore be naturally explained.
