There is a coefficient of bring-off-heat due to water evaporation whose value and position in calculation formula are different in various standards, when the enthalpy difference method is used for the thermal calculation of cooling tower. The paper presented suitable recommendations for the coefficient on definition, position and calculation formula, based on the theoretical analysis and comparison. The influence of the coefficient on the thermal calculation was studied, whose result showed that the impact of factor K could be ignored in the thermal calculation.