With the enhancement of people’s living standards, indoor air pollution attracts broad concern. The ventilation index (VI) began to be released to the public operationally from June 2006. By testing and analyzing 1790 data records, including pressure, temperature, humidity, wind, visibility, and weather phenomena, apparent temperature is selected as a main forecasting factor. Different forecasting formulas of summer VI in seven periods (05:00 to 08:00, 08:00 to 11:00, 11:00 to 14:00, 14:00 to 17:00, 17:00 to 20:00, 20:00 to 23:00, 23:00 to 05:00) are developed, considering the influences of summer unusual weather phenomena and wind speed, and the corresponding grades of VI are classified.