The pressure distribution and pressure gradient characteristics of shield screw conveyor directly affect its normal function. Taking the conditioned soil microelements in the spiral tube as the research object, the pressure field distribution and pressure gradient theoretical calculation model of the shield screw machine were established based on the stress analysis and the mechanical equilibrium conditions at steady state and the influence parameters were analyzed. The correctness and validity of the proposed calculation model are verified by the experimental results of the indoor model machine. Under the condition that the conditioned soil is the plastic fluid paste, the conclusion that the pressure field and pressure gradient of the shield screw machine decrease linearly along the direction of the screw axis is also obtained.