Abstract: A 40 kW injection locked gyrotron is designed based on theoretical calculations and simulations. The interaction structure is formed by two cavities which operate under TE₀₁-TE₀₃ mode pair. The state of the beam across the pre-bunching cavity is calculated with the fixed profile theory, and the operation parameters of the gyrotron under free oscillation state is calculated with the self-consistent theory. The regulation of the oscillation frequency varying with the parameters is achieved, based on which the locking bandwidth is proposed. A PIC code is used for simulating the injection locking state, a gain of 30.5 dB and a locking bandwidth of 20 MHz are achieved under a drifting length of 7 mm. If the drifting length or the input power is increased, the locking bandwidth could be wider. The calculations and simulations show that injection locking of the 40 kW gyrotron is feasible.