The invention provides a seed cotton moisture regain regulating system and method used before cotton ginning. The system comprises a first-level humidifying unit and a second-level humidifying unit. The first-level humidifying unit comprises a first-level humidifying induced draft fan, a first-level humidifying mixing box and a first-level humidifying tower which are sequentially connected in series through a main pipeline. The second-level humidifying unit comprises a second-level humidifying induced draft fan, an atomizer, a second-level humidifying mixing box and a second-level humidifying tower which are sequentially connected in series through a main pipeline. a first-level seed cotton unloader and seed cotton cleaning machines are sequentially connected in series between the first-level humidifying mixing box and the second-level humidifying mixing box through a main pipeline, moisture regain online detectors are arranged behind the first-level humidifying tower and the second-level humidifying tower respectively, and the second-level humidifying unit is arranged between a first-level seed cotton cleaner and a second-level seed cotton cleaner. The system and method remarkably improve the accuracy of seed cotton moisture regain real-time control in different technical processes, and meanwhile improve the processing quality of cotton.