In this paper, K2Ba［B4O5(OH)4］2·8H2O was prepared by solution method, and KBaB5O9 was prepared by heat treatment of K2Ba［B4O5(OH)4］2·8H2O. The samples were characterized by XRD, FT-IR, and TG-DTA-DTG. The phase change process in the preparation of KBaB5O9 by heat treatment of K2Ba［B4O5(OH)4］2·8H2O was analyzed and studied. The phase change experiences five stages: the loss of crystal water, the loss of hydroxyl, recrystallization, redecomposition, and recrystallization after melting. The loss of crystal water is divided into two steps. The kinetic parameters of the second step loss process of crystal water in K2Ba［B4O5(OH)4］2·8H2O were calculated by Kissinger method, Flynn-Wall-Ozawa method, and atava-estk method. The activation energy Es of the second-step loss process of crystal water in K2Ba［B4O5(OH)4］2·8H2O is 151.94 kJ/mol, the logarithm of pre-exponential factor lg As is 21.25 min-1, and the mechanism function is G(α)=(1-2α/3)-(1-α)2/3 (α is conversion rate).