A power conversion system and power control method for reducing cross regulation effect uses a voltage feedback adjustment circuit to modulate an error signal fed back from an output voltage so as to predict the energy of an output corresponding to its load states. While the energy delivered to an output terminal with its load remaining the same does not change, the energy delivered to an output terminal with its load changing is adjusted accordingly. The power conversion system thus effectively reduces the cross regulation effect and obtains excellent steady system output and transient response.