In this paper, the formation process of high-Tc phase in Bi-based supercondutor is investigated by X-ray diffraction etc. The high-Tc phase might be formed in the following manner. At first, the nuclei of hig-Tc (2223) phase are formed by the reaction of Sr-Ca-Cu oxide with low-Tc (2212) phase; Then the nuclei of 2223 phase grows, following the manner of ordinary crystal growth. There are two ways to form Sr-Ca-Cu oxide. One is accomplished by solid state reaction between Ca2CuO3 and Ca3(1-x)Sr3xCu5O8, and the other by liquid-state reaction of Ca2PbO4, 2212 phase and CuO. The liquid-state reaction rate is much quicker than that of solid state reaction. So, the rate of formation of 2223 nuclei can be accderated in liquid-state reaction. Based on this mechanism, we propose that Pb can only accelerate the nucleation of 2223 phase and exert no significant influence on the growth of 2223 phase.