Abstract: There are some technology problems of routine electrostatic precipitator, such as difficult charging of fine-particle dust, anti-corona, and dust re-entrainment. These technology problems limit the further improvement of dust removal efficiency. The efficiency deep improvement technologies of electrostatic precipitator were studied in detail. Firstly, the dust-removal efficiency deep improvement process was put forward, which included dust pre-collection in inlet smoke box, two-dimensional dust collection at the tail of electric fields, and dust anti escape technology in outlet smoke box. Secondly, the "W" shaped and trapezoid net-like dust collection devices perpendicular to the air flow direction were developed, which significantly increased the dust collecting area of electrostatic precipitator and solved the problem of dust re-entrainment. At last, the efficiency deep improvement technologies of vertical airflow electrostatic precipitator were applied on 630 MW coal-fired unit project. The application results showed that the dust concentration could be controlled to be lower than 10 mg/m³ at the outlet of electrostatic precipitator without changing the shape and size of the electrostatic precipitator and the number of electric fields. The dust concentration at the outlet of electrostatic precipitator was reduced from 14.4 mg/m³ before transformation to 6.5 mg/m³ by applying the technology. And the efficiency improvement range was as high as 54.9%, which was equivalent to that of the low-temperature electrostatic precipitation technology. Apparently, the dust concentration could achieve ultra-low emissions by applying the technology.