Marine sediment soft soil is one kind of typical structure soil with high sensitivity and low bearing capacity, widely distributed in the coast area of China. The In-situ mechanical properties of soft soil site are tested in the depth range of 29 meters through self-boring pressuremeter test. The in situ horizontal stress, critical plastic stress, undrained shear strength and shear modulus, etc. are obtaind. The curves of pressuremeter test show four stages of soil, including stress balance stage, elastic stage, plastic stage and failure stage. Unloading reduces the effective stress of soft soil significantly before the last stage; much attention should be paid to the stability of soft soil during unloading process in engineering practice. Based on the obvious nonlinear characteristic of soft soil’s shear modulus, it is more reasonable to get shear modulus by using nonlinear analysis method. On the basis of the experimental results, using fitting methods to set up empirical formulas of in situ mechanical properties and critical plastic stress varying with depth, so as to provide a reference instruction for the determination of the parameters in engineering.