In addition to the soft soil formed by natural settlement, with the development of social economy, the ultra soft soil represented by sludge and engineering waste slurry is widely distributed in cities. Due to the poor cohesiveness of ultra soft soil, soil particles will be in a floating or dispersive state when it has a high water content, showing a flow to high flow characteristics, and its strength and other engineering characteristics indexes are extremely low. In order to accurately determine the physical characteristics of ultra soft soil, the physical indexes and variability of soft soil in Tianjin are obtained by collecting the soft soil test results of hundreds of engineering projects in Tianjin. The regression equation is used to analyze the correlation of soft soil indicators in Tianjin, and the correlation between physical indicators is analyzed. The relationship between water content, density and void ratio of ultra soft soil is obtained, and its accuracy is verified by the physical indicators of ultra soft soil in various regions of China. The research results can provide basic support for the resource utilization of ultra soft soil, such as preparing muddy solidified soil.