At present, there are three algorithms for the design of the second flow rate of pipeline direct drinking water system: traditional formula algorithm, modified traditional formula algorithm and probability formula algorithm, and the calculation results of these three algorithms are compared and the reasons are analyzed. It is pointed out that the traditional formula algorithm and the improved traditional formula algorithm are not suitable for the pipe network calculation of pipeline direct drinking water system, and the probability formula algorithm is a more suitable method. The calculation of design second flow is the basis of hydraulic calculation of pipe network, and the correct calculation of design second flow can ensure the normal operation of the whole system. If the design second flow calculation is too large, it will lead to excessive pipe diameter and pump flow, resulting in economic waste; At the same time, the flow velocity in the pipe network is too small, which is easy to lead to bacterial reproduction and particle deposition. However, if the designed flow per second is too small, the selected pipe diameter will be too small, resulting in high head loss, waste of energy and serious water cut-off, which can not guarantee the reliability of water use. Therefore, it is very important to choose the correct design second flow calculation method.