The Tatun volcano group (TVG) proximate to the 7-million-population metropolis Taipei has long been a central concern in volcanic hazard in the Taiwan. While the TVG has been previously considered an extinct volcano, recent evidences suggested a much younger age of the last eruption event (~6000 years) and the possible existence of magma reservoir beneath the TVG. To examine and unveil the TVG magmatic plumbing system in detail, the local P-wave travel time data and the teleseismic waveform data from a new island-wide Formosa Array Project are combined for a 3D joint inversion. The new model reveals a pronounced magma reservoir with high fraction of melt (~16.5%) beneath the TVG at 5-25 km and better illuminate the deep magmatic structures that may suggest delamination processes in the northern Taiwan. Plain Language Summary The Tatun volcano group (TVG) is close to the metropolis Taipei and poses a direct threat to the 7 million population nearby. While the TVG was thought to be extinct in the past, recent studies found much younger eruption records (~6000 years) and some indications of alive magma chamber underneath. To better understand whether the magma chamber exists and what its geometry is, the Formosa Array project deployed 120 broadband stations in the northern Taiwan well covering the TVG area. Integrating the Formosa Array dataset, a new 3-D P-wave velocity model is constructed and shows a clear image of the magma chamber beneath the TVG at the depth of 5-25 km. Deep magmatic structures are also imaged and suggest the crust in the northern Taiwan was thickened and has possibly peeled off during the mountain building processes.