The invention discloses an underwater combined production conveying riser system and an installation method thereof. The system comprises a production riser assembly, a control riser assembly, a combined float bowl and a mooring device. The combined float bowl comprises outer float bowl bodies, inner float bowl bodies, external horizontal supports, internal horizontal supports, guide annular plates and limiting plates. The outer float bowl bodies are responsible for providing mooring rope tension, each inner float bowl body is responsible for providing top tension of a corresponding rigid production riser or top tension of a corresponding protective tube, decoupling of vertical stress and movement of the mooring device and all risers is achieved, decoupling of stress and movement of the risers are also achieved, and the in-place performance requirements of the rigid production risers are improved. Lower tube cable bodies of flexible control tube cables are assembled in rigid protective tubes in a unified and integrated mode, so that the complex galloping destruction problem can be avoided from being caused. The inner float bowl bodies can freely ascend and descend along sliding ways of the outer float bowl bodies, and thermal expansion requirements of the rigid production risers are met.