Abstract: The mathematical analysis shows that,with disregarding local head losses and by means of different simplified rigid models,the calculated results of the maximum pressure in a pressurized pipe system containing trapped air mass are equal to and independent of the initial length of the water-column.However,the calculation examples in this paper indicate that,if the initial water-column length is relatively short or the volume of the trapped gas is very small,the calculation error may be significant and even leads to a false conclusion.Therefore a complete rigid model is then presented in this paper,along with its theoretical limitation and suitable application terms.