The invention belongs to the technical field of geotechnical engineering soil tests and relates to a precision measurement test device and method for a Poisson ratio of a shallow soft sediment. The device comprises a pressure controller, a hydraulic controller, a computer, a triaxial apparatus, a volume compensation system and a high-precision deformation measurement system. The volume compensation system is arranged in the triaxial apparatus and is connected with the pressure controller. The high-precision deformation measurement system is arranged outside the triaxial apparatus and is respectively connected with the pressure controller and the hydraulic controller. The computer is respectively connected with the pressure controller and the hydraulic controller. The invention provides theprecision measurement test device and method for the Poisson ratio of the shallow soft sediment, transverse deformation and longitudinal deformation of a sediment soil sample can be measured in an in-situ stress state and the Poisson ratio is obtained, the disturbance to the soil sample is small, and the precision is high.