By thermogravimetry analysis (TGA), x-ray diffraction and R(T) relationship determination, the BiSrCaCuO system superconductors with composition of 2201, 2212 and 2223 were studied systematically. The experiments discovered that there is an unstable plane in 2223 sample, but not in 2201 and 2212. Comparing this result with the structure and property of 123 phase in YBCO-system, analyzing carefully, we found that only those superconductors which have the unstable plane are of higher Tc value. The role of the unstable plane in a superconductor is discussed. It is possible that 110K superconductivity in Bi-system has a close relationship with the existence of this unstable plane.