A majority of detection devices for detecting quality of indoor air, in particular toxic gas, are specialized instruments at present. These specialized instruments are expensive in price, inconvenient for carrying, not suitable for use of occasions with specific requirements or residential houses, and have no generality. On the other hand, a few apparatuses are gradually eliminated to cause waste of designs and resources due to single function. Owing to the above factors, a multifunctional portable air quality monitor based on an AVR (Automatic Voltage Regulator) single chip is designed. The monitor can provide two modes to monitor indoor gas concentration according to different requirements besides monitoring and detecting indoor temperature and humidity, alarm when the gas concentration is overproof (as to toxic gas or combustible gas) or lower than the standard (as to normal gas or gas required in special occasions), and prompt the user to pay attention to ventilating or detect air condition in the environment. In addition, the monitor has expansibility, so that the user can install various different gas sensors according to requirement, and set the sensor parameters according to an explanation of the apparatus and the monitor automatically identifies a new sensor. The monitor can be used for monitoring various different gases (in particular, toxic gas or flammable gas) because of the expansibility, and is suitable for various different occasions such as living rooms, workshops, mines, outdoors and the like.