Using liquid water as a proton source could greatly improve the economy of photocatalytic ammonia synthesis and environmental protection. However, the commonly used powder dispersion diphase system (liquid–solid) is seriously hindered by the scarce N₂ supply and the waste of photothermal energy. Here, a triphase reaction system (gas–solid–liquid) was constructed through a bismuth/carbon cloth (Bi/CC) float, which was self-supported on liquid water and directly exposed to a N₂-gas atmosphere. Assisted by the plentiful N₂ capture and efficient use of the photothermal effect of the CC for N₂ co-activation, this Bi/CC float triphase system breaks the N₂ concentration-dependent limitation in the traditional diphase system and exhibited a photothermal temperature (T)-dependent characteristic. A higher ammonia yield of 2.85 mmol L⁻¹ h⁻¹ g⁻¹ in the Bi/CC triphase system was achieved compared to that of 0.70 mmol L⁻¹ h⁻¹ g⁻¹ in a diphase system. Moreover, this triphase system also exhibited good stability and sunlight-driven feasibility, showing a real possibility for practical application in photocatalytic ammonia synthesis.