Abstract: As the technology of grid-forming (GFM) converters matures, offering advantages in maintaining stability of weak network, these converters will be widely applied to grid-connected units connected to new power system, such as flexible direct current transmission system and energy storage. Due to the difference in performance of GFM converters' strategy under distributed control, frequency measurement accuracy at each node, and spatiotemporal distribution characteristics of frequency, the long-time different output frequency between voltage source controlled-units (VSCUs) at both sides of AC transmission line will easily cause disharmony under normal operation. It is urgent to study whether such disharmony will further cause abnormal power oscillation. First, based on the analysis of different control strategies of GFM converters and influence factors of disharmony, an equivalent circuit model of an AC transmission line with disharmonious VSCUs is established. Then, to extend power flow analysis of new power systems, the formulas of the single-phase instantaneous current and power of the transmission line under disharmony conditions are deduced. The current and power peak gains are defined to measure the peak amplification effect of oscillation before and after disharmony. Finally, from the two aspects of synchronization capacity and harmony ability between VSCUs, the effects of line length, disharmony degree, and initial phase difference on the peak gain and the first appearance time of the maximum peak are analyzed. Furthermore, the influence of improving frequency measurement accuracy on disharmony oscillation is discussed. The results show that the disharmony oscillation cannot be eliminated only by improving self-synchronization capability between the GFM VSCUs, or GFM control accuracy, or frequency measurement accuracy.