Abstract: As for the planning Caohai section of Anning-Songming line which adopts shield tunnel project, the peaty soil is the main engineering geological problem. The sedimentary environments, distribution, characteristics and general physical and mechanical indices of the peaty soil are studied, and the special in dices of the second layer of peaty soil are analyzed. The second layer of peaty soil with large compressibility, low degree of consolidation, high shear strength and unconfined compressive strength, low sensitivity, low dynamic strength, large dynamic shear modulus ratio and small damping ratio has great influence on the shield tunnel project. Large settlement and differential settlement are easily caused, and the seismic performance is poor. Accordingly, the test schemes and measures should fully consider the particularity of the peaty soil.