Variable cycle engines (VCE) have always been a hot topic in aircraft engine research. Because of its variable thermal cycle characteristics, it has a wide range of optimum operating conditions (altitude, flight Mach number). In this paper, a typical double bypass variable cycle engine is used as the object of study, and the Gasturb11 software is used to simulate the engine in two main modes at different altitudes and Mach numbers, to observe the engine operation performance, as well as the relationship between the range and endurance from the derivation of cruise equation. The results show that VCE has a best thermal cycling pattern for a given altitude and Mach number, and for a range of lift-to-drag ratios, resulting in the longest range or endurance of the aircraft. This paper investigates the relationship between range, endurance and thrust of a variable cycle engine and altitude and Mach number to help make the VCE better suited to the needs of an aircraft with multiple missions, long range and high endurance.