Helicopter flight test is an important means to verify whether the flight performance meets the requirements of airworthiness regulations under the environmental conditions close to the maximum altitude limit. During the performance flight test at high plateau, the helicopter is near the using limitation, the flight risk is high. It is necessary to study and develop reasonable and safe test flight methods to obtain accurate helicopter performance data while ensuring test flight safety. This paper studied the requirements of airworthiness regulations for high plateau test flight, determined the test flight method of each test flight items, and applied the test flight methods to complete the H425-100 helicopter high plateau test flight at Yushu Batang airport at 3950m above sea level. It shows that the testing methods determined in this paper is reasonable and feasible. Tethered hover method can improve the efficiency of test flight. Using OEI simulating mode can effectively reduce the risk of test flight. The fitting method can be used to determine the helicopter take-off and landing distance more accurately.