The invention provides a slurry balanced shield slurry density detecting device and detecting method; a slurry feed pipeline and a slurry discharge pipeline of shield slurry are formed by connecting horizontal pipes and vertical pipes at intervals, and pressure sensors for detecting the pipe internal differential pressure of the horizontal pipes and the vertical pipes are arranged on the slurry feed pipeline or the slurry discharge pipeline respectively, and an 'inverted-L' shaped differential pressure differentiator is formed, to eliminate errors of various dynamic hydrodynamic disturbance factors on gravity method detection of the pipeline slurry density; the technical problems in the prior art of extremely large detection errors caused because a static slurry density formula is simply applied for measurement of the density of slurry with a certain flow quantity by using a gravity theory, and time and labor consuming and no convenience caused because a gamma densitometer has complicated maintenance and management and the the like are solved.