In the traditional power system, the synchronous generator independently forms the internal voltage amplitude/frequency, which is connected to grid to establish the system voltage. However, the establishment of grid voltage at present is increasingly dependent on renewable energy equipment. Under the phase-locked synchronization, the grid-connected converter needs to detect the grid voltage to form the internal voltage, which seems to be different from the synchronous machine that independently forms the internal voltage and further establishes the system voltage. On this basis, the mechanism of internal voltage amplitude/frequency formed by the current control of the phase-locked synchronous converter is studied, and it is explicitly stated that the internal voltage amplitude/frequency is uniquely determined by current, which is essentially the same as the synchronous generator independently forming the internal voltage to establish the system voltage. In this paper, based on the control structure and the nature of forming the AC instantaneous value, it is explained that the converter output is essentially the internal voltage amplitude/frequency. Then, starting from the closed-loop dynamic process of equipment and network, the redundant relationship between terminal voltage and current is clarified. After the input current is determined, the internal voltage amplitude/frequency can be uniquely determined accordingly. Finally, combined with the simulation analysis, the correctness of uniquely determining the internal voltage amplitude/frequency by input current is demonstrated.