This paper gives a description of the simulation teats for underwater operating conditions of a four-stroke turbocharged diesel engine. The test results show that under the operating conditions at a given high backpressure, high vacuum and fluctuating backpressure, the turbocharged diesel engine can run smoothly and reliably, thus furnishing a basis for extending the application range of turbocharged diesel engines. After more than 1300 hours of simulation tests for under water operating conditions, the following results have been obtained: 1) The cold starting of the engine with the exhaust main closed can be realized without any difficulty. 2) The test engine on underwater operating conditions can develop a maximum continuous power of 1765 kW corresponding to a mean effective pressure of 11.5 bar (11.7 kg/cm ). 3) The engine can operate with or without load through discharging water by exhaust gases. 4) When the exhaust backpressure fluctuates at an amplitude of 0.28 bar (0.29 kg/cm²) the engine runs properly. It is concluded that so long as the design and adjustment are properly done it is possible for a turbocharged diesel engine to run steadily and reliably under given high backpressure, high vacuum and fluctuating backpressure. Even though the scavenging process at low load badly deteriorates, the working points of the compressor are not in the surge region yet. Moreover, under various underwater operating conditions, a four-stroke turbocharged diesel engine is still characterized by higher thermal efficiency and favourable economy. It is held, therefore, that the application of four-stroke turbocharged diesel engines in underwater engineering has not only superiority, but also feasibility and the tests described in this paper have provided the basis for this application.