The particularity of the altitude simulated test of an engine with Laval nozzle was analyzed in brief.The engine thrust determination and correction in altitude test with large deviation of simulated ambient pressure was put forward.The test result shows that maximum tolerance of the simulated ambient pressure deviation is about 14% under which the engine thrust uncertainty remains less than 1.0%.Where there is a limitation of the test facility capability or for other reasons the large deviation of simulated ambient pressure may occur.The presented method can be used to determine the thrust of the engine with Laval nozzle.