Residents living in the cold areas such as Harbin generally experience a residence time of approximately 6 months in chilly winter without frequent natural ventilation. To find out the influence of a short period of ventilation on the indoor formaldehyde concentration inside a new building, an investigation was conducted for the instance of twice ventilation in a day through window opening. The results showed that the initial concentration of formaldehyde was 3.53 - 8.48 times as high as the concentration after 10 min ventilation. After closing the window, the indoor formaldehyde concentration increased with time and followed an exponential equation of C = C0exp( - b * t) + (a + Cw) [ 1 - exp( - b * t) ] with correlation coefficient (R2) of 0. 945 -0. 999, based on the statistical analysis of 14 groups of measurement data. The developed equation can be used to estimate the emission rate of indoor formaldehyde sources and the air exchange rate of the test room simultaneously.