We report the first maximum-light far-Ultraviolet to near-infrared spectra (1000A - 1.62um, rest) of a H-poor superluminous supernova, Gaia16apd. At z=0.1018, it is one of the closest and the UV brightest such events, with 17.4 (AB) magnitude in Swift UV band (1928A) at -11days pre-maximum. Assuming an exponential form, we derived the rise time of 33days and the peak bolometric luminosity of 3x10^{44}ergs^-1. At maximum light, the estimated photospheric temperature and velocity are 17,000K and 14,000kms^-1 respectively. The inferred radiative and kinetic energy are roughly 1x10^{51} and 2x10^{52}erg. Gaia16apd is extremely UV luminous, emitting 50% of its total luminosity at 1000 - 2500A. Compared to the UV spectra (normalized at 3100A) 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 56Ni, 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 >=90Msun, where substantial 56Ni material is produced. Higher photospheric temperature of Gaia16apd than that of normal SNe may also contribute to the observed far-UV excess. We find some indication that UV luminous SLSNe-I like Gaia16apd could be common. Using the UV spectra, we show that WFIRST could detect SLSNe-I out to redshift of 8.
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