Controlling the phase and frequency of a gyrotron backward-wave oscillator (gyro-BWO) by means of injection-locking techniques is of practical importance. Using a nonlinear self-consistent time-independent code, this study analyzes the stability of a gyro-BWO with an external injection signal. To examine the stability of steady-state solutions, the perturbation of the phase difference between the oscillator and injected signal must decay in time.