The 120 mm forward loading mortar is equipped with a mechanism enabling the launch platform to be flipped backwards to the ground. This has the advantages of small space occupation, high shooting accuracy, and good consistency. However, the mechanism used for the launch platform has not been systematically studied. To address this issue, this article uses finite screws to synthesize the mechanism configuration based on the 3-DoFs (three degrees of freedom) motion requirements of the barrel. In order to meet the requirements of foldability and firing accuracy, a number of feasible limbs for 1T2R (T-translational, R-rotational) parallel mechanisms with two limbs and the fewest joints have been generated. One of these mechanisms has been selected for detailed analysis. The closed-loop vector method and numerical method are employed to investigate the relationship between input and output under boundary conditions, thereby providing a theoretical and technical foundation for the development of 120 mm mortars.