A reconfigurable modular robot is proposed, which can change its configurations to improve its stability and anti-tipover capability. This three-module tracked robot has three kinds of symmetrical configurations, that is, line type, triangle type and row type. Based on analyzing the factors and countermeasures of mobile robot tipover problem, a stability pyramid and the tipover stability index are proposed to determine globally the mobile robot's static and dynamic stability. And the tipover stability index and global stability of the 3 symmetrical configurations of the metamorphic robot under the combined disturbance of pitch, roll and yaw, are discussed. The simulation and experiment in unstructured environment are presented.