The equilibrium distribution of a polymer chain between two interconnected spherical cavities (a small one with radius Rs and a large one with radius Rl) is studied by using Monte Carlo simulation. A conformational transition from a double-cavity-occupation (DCO) state to a single-cavity-occupation (SCO) state is observed. The dependence of the critical radius of the small cavity (RsC) where the transition occurs on Rl and the polymer length N can be described by RsC∝N1/3Rl1-1/3ν with ν being the Flory exponent, and meanwhile the equilibrium number (ms) of monomers in the small cavity for the DCO phase can be expressed as ms = N/((Rl/Rs)3 + 1), which can be quantitatively understood by using the blob picture. Moreover, in the SCO phase, the polymer is found to prefer staying in the large cavity.