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A spherical metamorphic mechanism with parallel structure was proposed. It consisted of a moving platform, a base plate and four connecting legs between moving platform and base plate. Three of these legs had the same structure named circle legs and the other one named middle leg. Spherical metamorphic mechanism could transform to normal configuration and metamorphic configuration respectively with the change of constraint applied by middle leg. The moving platform was in possession of three revolute degrees of freedom which equivalent to spherical joint and extra translational freedom in the direction of radius. Based on screw theory, the mobility of this mechanism was analyzed firstly. Secondly, the constraint sub-matrix and actuation sub-matrix were obtained and the complete Jacobian matrix was established by integrating these two sub-matrices. In the end, the singular conditions of spherical metamorphic mechanism were analyzed by investigating the ranks of the complete Jacobian matrix and methods used to avoid the occurrence of singularities were given which were propitious to mechanical design.