Abstract: The ionospheric Total Electron Content (TEC) is one of the key parameters of ionospheric sounding results. As the main input parameter, the measurement precision of TEC will directly affect the Computer ionospheric Tomography (CT) imaging result. In the past, the dual-band technique was mainly used for TEC measurement. However, due to the solution of the phase integral constant and error of the system hardware delay, the TEC results cannot satisfy the high accuracy requirement of reconstruction imaging by using the ionospheric CT. The newly developed tri-band technique throws new light on the space-based ionospheric sounding techniques. A new method is proposed for inversing TEC on the basis of combining the propagation delay and phase with the tri-band beacon. By using the fusion of advantages of both detecting random electron density fluctuations and computing phase integral constants with the tri-band beacon, it further improves the measuring resolution of the ionospheric TEC. Simulation results show that the CT resolution is improved by using the three-frequency measurement based on this method.