The outlet of tube gas screen protection technology was installed on the submarine platform to reduce effect of crosscurrent to underwater launch of missiles, this protection improve missiles mechanical environment underwater. In order to get the exact region of gas protection screen outside the tube, engineering calculation method to calculate the the upper and lower boundaries of protection system on the tube was obtained by theory and experiment analyze. The method was used to calculate upper and lower boundaries of the gas screen, the result agree well with the test result, so this method can direct the design of gas screen protection system.