Stimulus-responsive hydrogels are assembled into soft devices that transform their shape upon external stimuli. It is very important to understand the macroscopic assembly mechanisms and to modulate the interface stability of assemblies. In this study, polyelectrolyte hydrogels with outstanding mechanical performances and opposite charges were assembled into soft devices via electrostatic association. The interface strength could be tuned by varying the net charge density, which depends on the concentration of charged/chargeable monomers, the pH of the buffer solution, and the ionic strength of the solution. We propose that charge screening at the interface by free counterions causes a reduction of interface strength, whereas charge redistribution is helpful to strengthen the interface of the assemblies. The understanding of macroscopic assembly mechanisms provides significant guidelines for designing novel soft transducers and drivers and engineering their interface strength.