In the construction of shield tunnel, the strength of reinforced soil in the front of departure shaft concerns directly the security of construction. Based on Mechanics of Elastic Structures theory, a thin plate model about reinforced soil is introduced, the maximum stress on reinforced soil is calculated. The security of strength in reinforced area is discussed. With the derived formulas, the needed bending and shear strength of reinforced soil in the project of Beijing Metro Line No. 4 is calculated beforehand, and appropriate material and construction parameters are selected in reinforcement according to the calculate result.