Expanded graphite is used as framework material in inorganic or organic heat storage materials commonly, of which the function is to improve thermal performance of the relevant materials. However, its negative effect is unclear. In this paper, the change of endothermic temperature of chemical heat storage material Ca_xZn_y(OH)_2(x+y) by graphite addition is reported. Differential scanning calorimetry (DSC) is employed to perform the measurement. The result shows that the initial temperature and width of the endothermic peak decrease due to the action of the added graphite slightly; Along with the increase of graphite content, the endothermic peak moves to lower temperature further, the width and height of the peak reduce simultaneously.