Chamber was designed to simulate indoor environment to measure the formaldehyde emission of medium density fiberboard(MDF). The effects of thickness and sealed method of MDF, and temperature, relative humidity, air exchange rate of chamber on the formaldehyde emission of MDF were investigated. The mechanism of formaldehyde emission of MDF was discussed. Results show that formaldehyde emits from the edge of MDF mainly, the initial formaldehyde emission is nearly 1 times of that from the surface of MDF. Thinner MDF has greater formaldehyde emission. With the increase of temperature and relative humidity of chamber, the formaldehyde emission of MDF is increased. With the rise of air exchange rate of chamber, the formaldehyde emission of MDF is diminished. The process of formaldehyde emission of MDF consists of three major stages, i. e., fast emission in short term, slow emission in mid term and stable emission in long term.