Second harmonic gyrotron backward-wave oscillator (gyro-BWO) is a promising tunable source with only half the magnetic field requirement. But these merits have long been impeded by the transverse and axial modes competition as a result of lower efficiency. This study employs a slotted structure as a mode selector for suppressing the persistent low order transverse mode. In addition, an optimized two-step tapered structure is adopted to stabilize the high order transverse modes and axial modes. The interaction efficiency and the tuning bandwidth were greatly enhanced in the meanwhile. As a calculated result, a stable, ka-band, slotted second harmonic gyro-BWO is capable of producing an efficiency of 23% and a 3-dB tuning bandwidth of 9% at 5 A and 100 kV.