SThe prefabrication and assembly of biological reaction tank in sewage treatment plant is complex, and cracks may occur due to improper construction of ultra-high performance concrete (UHPC) wet joint connection. Combined with engineering examples, the shrinkage mechanism of UHPC is analyzed. By studying the influence of different components on the shrinkage of UHPC, the method of double mixing fly ash and expansive agent is proposed to effectively control the shrinkage deformation of UHPC and provide a strong guarantee for the crack control of on-site UHPC construction.