The invention relates to a shield and shield tail synchronous grouting slurry buoyancy measuring device and a measuring method thereof. The measuring device comprises a container with an open upper portion, a tension meter which is arranged above the container through a support and a duct piece model which is completely immersed in a slurry, wherein the slurry is accommodated in the container; twoends of the duct piece model are covered by sealing films; and the duct piece model is connected to a lower end of the tension meter. A cross rod is erected above the container so as to arrange the tension meter. Through measuring gravities of the duct piece model in two states of absence of the slurry and immersion in the slurry respectively, upper buoyancy of the duct piece model is acquired. Aproblem that the upper buoyancy of the duct piece model can not be accurately measured in the prior art is solved.