A special instument is presented for monitoring the transient process in a turbojet engine, which is called "transient operating line indicator". The instrument consists of analog computing circuits. It demonstrates the real-time flowrate and the pressure ratio of the engine which are calculated from the total pressure and the static pressure at the inlet and the total pressure at the exit of the compressor. By means of this instrument the transient operating line of the engine can be plotted with an x-y recorder. The description, the circuits and the performance of this instrument are given in this paper. The various transient processes of turbojet engine, such as acceleration, deceleration and surge, have been studied with this instrument.