The co-catalyst strategy for enhancing photocatalytic efficient nitrogen reduction reaction (NRR) for ammonia synthesis has attracted great attention, but it faces problems such as expensive precious metals, low efficiency and poor stability. In this paper, WSe2 nanoflowers with novel semi-metallic 2 M phase as co-catalysts was loaded on g-C3N4 nanotubes (NTs) (2 M WSe2/g-C3N4 NTs) by solvothermal method and simple mechanical grinding. The optimized 2 M WSe2/g-C3N4 NTs shows improved photocatalytic NH4+ generation rate (9.35 mM·h−1·g−1) and good stability, which is 2.4 and 1.6 times higher than g-C3N4 NTs and Pt/g-C3N4 NTs. The apparent quantum efficiency (AQE) of 2 M WSe2/g-C3N4 NTs with 0.02 g catalyst is 2.58 % under 370 nm. The increased activity of 2 M WSe2/g-C3N4 NTs is attributed to the following factors: (1) 2 M WSe2 nanoflowers with novel semi-metallic 2 M phase is composed of nanosheets, offers a larger active surface area and exhibits excellent conductivity, which is attributed to the few-layer structure typical of two-dimensional transition metal selenides of 2 M WSe2; (2) 2 M WSe2 is loaded on the surface of hexagonal g-C3N4 NTs in the form of typical few-layer nanosheets, which can expose more active Se sites for N2 reduction; (3) DFT calculations show that the N2 fixation of g-C3N4 and 2 M WSe2/g-C3N4 proceeds through alternating pathways at the C and Se sites, respectively, which is favorable for the formation of *NH-NH2 intermediates, thus accelerating the hydrogenation process. The novel semi-metallic 2 M WSe2 as co-catalyst provides insights in the development of efficient and environmentally friendly NRR photocatalysts.