Abstract: The X-band transit time tube oscillator (TTTO) is a novel high power microwave device, simulation shows that it frequency-spectrum characteristic is related with its output, so the frequency spectrum diagnosis is an important work for the X-band TTTO research. Using t he frequency synthesis method, the frequency-spectrum characteristic of the X band TTTO is got with the system. Based on the frequency-spectrum characteristic, the structural parameters and working parameters of the X-band TTTO are adjusted to get better output.