We report the first maximum-light far-Ultraviolet to near-infrared spectra ( 1000Å - 1.62 \mu m , rest ) of a hydrogen-poor superluminous supernova , Gaia16apd . At z = 0.1018 , it is the second closest and the UV brightest SLSN-I , with 17.4 magnitude in Swift UVW2 band at -11 days pre-maximum . The coordinated observations with HST , Palomar and Keck were taken at -2 to +25 days . Assuming an exponential ( or t ^ { 2 } ) form , we derived the rise time of 33 days and the peak bolometric luminosity of 3 \times 10 ^ { 44 } erg s ^ { -1 } . At maximum , the photospheric temperature and velocity are 17,000 K and 14,000 km s ^ { -1 } respectively . The inferred radiative and kinetic energy are roughly 1 \times 10 ^ { 51 } and 2 \times 10 ^ { 52 } erg . Gaia16apd is extremely UV luminous , emitting 50 % of its total luminosity at 1000 - 2500 Å . Compared to the UV spectra ( normalized at 3100Å ) of well studied SN1992A ( Ia ) , SN2011fe ( Ia ) , SN1999em ( IIP ) and SN1993J ( IIb ) , it has orders of magnitude more far-UV emission . This excess is interpreted primarily as a result of weaker metal line blanketing due to much lower abundance of iron-group elements in the outer ejecta . Because these elements originate either from the natal metallicity of the star , or have been newly produced , our observation provides direct evidence that little of these freshly synthesized material , including ^ { 56 } Ni , was mixed into the outer ejecta , and the progenitor metallicity is likely sub-solar . This disfavors Pair-Instability Supernova ( PISN ) models with Helium core masses \geq 90 M _ { \odot } , where substantial ^ { 56 } Ni material is produced . Higher photospheric temperature definitely contributes to the far-UV excess from Gaia16apd . Comparing with Gaia16apd , we find PS1-11bam is also UV luminous .