The invention relates to a method for designing a longitudinal differential settlement tolerance value of an immersed tube tunnel, which comprises the following steps of: 1, establishing a longitudinal calculation model; 2, calculating a tolerance shearing force Q at the position of a certain joint; 3, calculating settlement delta and a foundation stiffness k of a random spring; 4, calculating a longitudinal interval length L0 with influence on the shearing force of the joint; 5, obtaining a stiffness change amplitude [alpha] and a change pattern factor beta of an average foundation stiffness of all the springs in the range of L0 and drawing a stiffness change distribution diagram; 6, drawing a joint shearing force influence line and multiplying the shearing force influence line by the stiffness change distribution diagram to obtain a relation of the tolerance shearing force of the joint and the stiffness change amplitude as well as the pattern; 7, deriving a relation of tolerance differential settlement at both ends of the L0 and the stiffness change amplitude; 8, substituting a formula in the step 6 into the step 7 to obtain a longitudinal differential settlement tolerance value calculation formula; and 9, analyzing actually generated differential settlement delta at both sides of the inspected joint and when the condition that delta is less than or equal to [delta] is met, illustrating that the calculated longitudinal differential settlement meets the requirement on stress of a shear key of the joint.