How to realize self-recovery after flipping is as important as how to enhance the ability to overcome obstacles for rescue robots. In order to meet both requirements, a new structure of mechanical legs need to be brought forward. Based on traditional mechanisms, a new kind of 3-RPS/(H) metamorphic parallel mechanism is proposed in this work. Different from 3-RPS mechanism, the mechanism comprises a novel intermediate metamorphic branch. The different constraints provided by the intermediate metamorphic branch and the locking device add flexibility to the metamorphic parallel mechanism. The working modes of the 3-RPS/(H) metamorphic parallel mechanism were analysed, and their degrees of freedom were obtained. An inverse kinematic analysis of the mechanism was also performed, and its workspace was studied in comparison with that of a traditional 3-RPS parallel mechanism having the same parameters. The results showed that the 3-RPS/(H) metamorphic parallel mechanism had better flexibility than the 3-RPS mechanism. The influences of various structural parameters of the metamorphic parallel mechanism on the workspace were analysed. Finally, the singularity analysis for the mechanism is brought forward. The results are expected to provide a theoretical basis for the design and optimisation of mechanical legs.