This paper discusses dynamics analysis and simulation of a miniature reconfigurable robot for indoor surveillance. The robot keeps spherical shape for overall protection during falling and transforms to a two-wheel mobile robot after landing. Due to the mass center of the robot is near to the wheel axis, the mobile performance is undesirable, and thus a retractable tail is mounted on the robot to improve the performance. In this paper, dynamics equations of the robot based on Lagrangian function are deduced, and discussions about dynamics analysis of the robot with and without the tail are also presented. Both simulations and experiments prove positive influence of the tail on the robot performance.