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We report on XMM-Newton/Chandra/Swift/HST observations of the ultraluminousX-ray source (ULX) in NGC 247, which is found to make transitions between thesupersoft ultraluminous (SSUL) regime with a spectrum dominated by a cool($\sim 0.1$ keV) blackbody component and the soft ultraluminous (SUL) regimewith comparable luminosities shared by the blackbody and power-law components.Multi-epoch observations revealed an anti-correlation between the blackbodyradius and temperature, $R_{\rm bb} \propto T_{\rm bb}^{-2.8 \pm 0.3}$, rulingout a standard accretion disk as the origin of the soft X-ray emission. Thesoft X-ray emission is much more variable on both short and long timescales inthe SSUL regime than in the SUL regime. We suggest that the SSUL regime may bean extension of the ultraluminous state toward the high accretion end, being anextreme case of the SUL regime, with the blackbody emission arising from thephotosphere of thick outflows and the hard X-rays being emission leaked fromthe embedded accretion disk via the central low-density funnel or advectedthrough the wind. However, the scenario that the supersoft ULXs are standardULXs viewed nearly edge-on cannot be ruled out. Flux dips on a timescale of 200s were observed. The dips cannot be explained by an increase of absorption, butcould be due to the change of accretion rate or related to thermal fluctuationsin the wind or disk. The optical emission of NGC 247 ULX exhibits a blackbodyspectrum at a temperature of 19,000 K with a radius of 20 $R_\odot$, likelyarising from an OB supergiant companion star.

Nature of the soft ULX in NGC 247: super-Eddington outflow and transition between the supersoft and soft ultraluminous regimes