Some special tasks require human operations and cannot be performed by robots or other autonomous equipment, such as special industrial assembly and surgical procedures in small and crowded spaces. Workers/surgeons in these cases tend to have physical fatigue. In this study, a novel variable-stiffness joint based on positive pressure was proposed, and a torque model was established. The locking torque variation, step response, and energy consumption were evaluated in comparison with a torque motor. A lockable lower-limb exoskeleton based on the variable-stiffness joint was developed, and wearable tests were conducted to evaluate a voice recognition interface and supporting performance. The locking torque of the variable-stiffness joint could be continuously varied from 0 Nm to 26 Nm with the air pressure ranging from 1.6 bar to 5.5 bar. The settling time was 0.328 s in the step response experiment. With a load of 6 Nm, the variable-stiffness joint can realize an energy consumption reduction of 75.01% compared with using a torque motor. Moreover, the lockable lower-limb exoskeleton can realize a 35–60% reduction in the average muscle activation in each subject (aged 22–57) to maintain squatting postures at three different knee angles (paired t-test, P < 0.01). The proposed exoskeleton system has good mobility, low energy consumption, and easy-to-control features, showing great potential in supporting the weight of workers/surgeons during long-term operations.