Abstract: According to the practical working principle of vertical inverted U-shaped tube natural circulation steam generator（SG）,mathematic model of the number of impurities from the second side of SG was established,and the specific functional relationship between the number of insoluble impurities from the second side of SG and the water and sewage flow was given.The result indicates that the number of insoluble impurities from the second side increases on the basis of the exponential function,with certain limit value,under the condition that the continuous operation of SG reaches around 15 full power days,the number of insoluble impurities from the second side is close to the maximum value,and the emission of impurities of the sewage pipe increases with the increase of the density（amount） of impurities from the second side,until it nearly equals the amount of impurities entered into SG.