Beam misalignment effects on a high-frequency second harmonic 0.8 THz gyrotron tube installed in a 15T magnet are studied comprehensively in this article. Beam alignment is critical as the cavity in the gyrotron tube at this frequency is typically about 3 mm in diameter. and the tube is more than 1 m long. Starting current rise, efficiency degradation, continuous frequency tunability and harmonic mode competition are all analyzed both from a theoretical basis and by experimentation for a variety of beam misalignments. The gyrotron tube alignment process is also discussed to demonstrate how to achieve good alignment of the electron beam in the cavity.