The invention discloses an automatic flattening device for a carding machine of an electro-mechanical room. The automatic flattening device comprises an airflow detection mechanism, a feeding mechanism and a flattening device. The feeding mechanism comprises a first horn seat, a second horn seat and two compaction rollers. The first horn seat and the second horn seat are in the same structure. A feeding port is arranged between the first horn seat and the second horn seat. One end of the first horn base and one end of the second horn base are connected with a feeding groove. The airflow detection mechanism is composed of two compression seats. The surfaces of the compaction rollers are connected to arc seats. The arc seats annularly rotate on the surfaces of the compact seats through groove structures. One end of each arc seat is connected with a universal seat. The flattening device comprises transfer rollers, cotton rolls, and a discharge groove. The transfer rollers are arranged at bottom ends of sliding grooves. The cotton rolls and the transfer rollers are all arranged at the upper end of the discharge groove. The airflow detection mechanism is used for detecting the number of cotton sliver so that the size of a feeding opening is reasonably selected. The automatic flattening device is simple in overall structure and high in flattening efficiency and not easily damaged, which is worth promoting.