The work aims to design a general underactuated flexible manipulator, with respect to the problem that the object to be grasped in the light industry and food industry has various physical properties and a large size range. The manipulator structure was designed according to the function that the manipulator needed to meet. The Fsolve function in Matlab software was used to solve the relationship between the relative rotation angle of the middle finger and fingertip of the manipulator and the pressure of the pen-shaped cylinder, based on which the workspace drawing of the manipulator was drawn. The simplified grab model of the manipulator was established, and the grab calculation of the manipulator was carried out with 500 mL beer and 160 mL cola as the grab targets. The size adaptive continuous width range of the manipulator was 0-200 mm, which could accurately control the contact force between the manipulator and the object while adapting to the size of the object to be grasped. The designed manipulator has a large grab range, and the series double hinge structure installed with torsion spring enables the manipulator to achieve reliable grabbing of the target without sensors.