The service tunnel of Xiang’an subsea tunnel in Xiamen has nearcircular crosssection. The area of the excavation crosssection of the service tunnel is 46m2. In the coastal land portion of the service tunnel, there exists 1000m long alternative ground consisting of fullyweathered soil layers and fullyheavily weathered rock and soil. The cover of the service tunnel ranges from 6m to 40m. The total length of the service tunnel is located below the ground water level. Due to the influence of the heavy rainfall in Xiamen area, the service tunnel has abundant ground water. The ground of the service tunnel may collapse when water is encountered. In the paper, expatiation is made on the safe and rapid construction of the service tunnel located in the abovementioned ground. Conclusion is made on basis of engineering experience that top heading/bench excavation method, whether the invert part is reserved or not, is favorable for the construction of the circular small crosssection tunnels. Furthermore, proper primary reinforcement parameters, construction step lengths and equipment matching are provided for the service tunnel constructed by means of top heading/bench excavation method. The paper can provide reference for similar works in the future.