Binarity is one of the essential features to understand massive star formation. Almost all massive stars are formed with bound stellar companions, but detecting massive protobinaries while they are still highly embedded and actively accreting are challenging. We report a new discovery of such an early-stage highly-embedded massive protobinary system at the center of the massive star-forming region G339.88-1.26 with latest ALMA observations. With an apparent separation of 170 au, it is one of the most closely separated embedded system identified so far. In this system, while source A has started photoionization and is therefore much brighter than source B, it is source B that dominates the accretion and outflow activities. Various molecular emission lines show rotation signatures toward source B. By modeling the kinematics of molecular line emission/absorption, as well as hydrogen recombination lines, we found that, source A, with a highly confined photoionized region, is embedded inside the source B disk. Source B is also driving a wide-angle rotating SiO outflow with an angular momentum of ~2000 au km/s and estimated launching radius of ~ 20 au. The SiO outflow becomes more collimated at larger distances and connects to the large-scale CO and SiO outflows observed previously. Furthermore, about 20 fainter compact continuum sources are identified in the surrounding 20,000 au region, clustering toward the central massive binary system. This newly discovered system provides valuable insights for understanding how massive binaries form and accrete. It appears to be consistent with the prediction of disk fragmentation scenario of binary formation that the secondary source could dominate the accretion which eventually leads to a near-equal-mass system. Another possibility is that, source B is actually more massive, but still in its bloated phase due to the high accretion rate and therefore has minimal photoionization.