Cleanout service plays an important role in hydrological well completion. With an example of packed backpressure cleanout, well cleanout by water hammer effect is elaborated in this paper. It is showed that by the model simplification, the water head difference caused by water hammer effect can reach 136m; the good effect of well washing is proved in theory. Based on the analysis on variance in the water hammer pressure formula, 3 methods for well cleanout optimization by water hammer effect are put forward: put suction nozzle of pump at the bottom end of packed interval, increase the pump volume or shorten the length of packed interval.