<p>With the rapid development of coastal and ocean engineering in recent years, many civil engineering constructions have encountered a type of soil with special properties known as ultra-soft soils, such as the regulation and utilization of mudflats, offshore wind power foundations, and submarine energy transmission pipelines. However, the current understanding of the engineering properties of ultra-soft soils is still incomplete, and there has been limited discussion on the property differences between typical ultra-soft soils and soft soils. This study systematically investigated the physical, strength, and deformation properties of typical ultra-soft soils sourced from the Kaolin soils, mudflat soils of coastal cities, and seabed surface fine-grained soils. The results showed that the variation trend of shear strength undergoes obvious changes after the water content of soil samples exceeds 1.5 times the liquid limit. This finding establishes an important basis for understanding the key engineering properties of ultra-soft soils, indicating that they are softer than common soft soils and have higher water content. Following this discovery, the physical indicators and deformation indicators of typical ultra-soft soils were discussed and compared with soft soils. Finally, a property indicator table to illustrate the differences between ultra-soft soils and soft soils was recommended.</p>