In this paper, a three-rotor turboprop engine (with a fixed-speed free turbine) is set as the studied object. Under the certain flight conditions and the transient state control law, the main influencing factors affecting the position of low-pressure rotor transient state line are studied. They are the ratio of MIL (moment of inertia of low pressure rotor) to MIH (moment of inertia of high pressure rotor), the ratio of HPC (high pressure compressor) pressure ratio to LPC (low pressure compressor) pressure ratio and the ratio of design point HPR (high pressure rotor) speed to design point LPR (low pressure rotor) speed. The influence of these three factors on the position of low-pressure rotor transient state line is obtained and the mechanism is analyzed.