Where urban traffic infrastructures crossing rivers or gulfs are to be built, it is common to make decisions between the bridge alternative and the underwater tunnel alternative. The article makes analysis and comparison on the bridge alternative and the underwater tunnel alternative in terms of construction cost, durability, security, ecological and environmental impacts and social efficiency, which concludes that the underwater tunnel alternative has more comprehensive advantages than the bridge alternative. The underwater tunnel alternative has the following advantages: the waterway transportation will not be affected during the operation of the tunnel; the traffics through the tunnel will not be affected by bad weathers and allweather transportation can be guaranteed; less land occupation and less existing infrastructure dismantling; the original natural scene of the water area can be reserved; the municipal pipelines crossing the rivers or gulfs can be installed effectively; higher capacity against natural disasters and wars.