The heat transfer coefficient of composite wall with fly ash blocks sandwich were studied. The heat transfer coefficient of four forms of wall specimens were determined. Based on the basic mechanism of the thermal conductivity of the four wall types:(1)clay bricks wall; (2)clay bricks wall with fly ash blocks sandwich; (3)recycled concrete bricks wall; (4)recycled concrete bricks wall with fly ash blocks sandwich, the relationship between coefficient of thermal conductivity of various materials and temperature,humidity could be derived,and an actual value calculation method for calculating the heat transfer coefficient that suitable for engineering design was also proposed. By analyzing the experimental values,theoretical values and the actual values of the specimens,the actual value calculation method was proved to be correct and reasonable; using recycled concrete bricks instead of clay bricks in composite wall with fly ash blocks sandwich the good insulation effect can also achived, which contributes to a building energy efficiency design detailing on using recycled concrete bricks and composite wall with fly ash block sandwich.