Indoor air quality monitoring is of great importance to human health as people typically spend more than 90% of their time in indoor environments. An indoor air quality detector (IAQD) enabling measurement of CO2, PM2.5, temperature, humidity, has been designed and tested in residential buildings based on Internet of Things (IoT). The hardware and software design of IAQD are described in detail, and seven IAQDs with Zigbee wireless module embedded were deployed in the target building for a continuous period of one month in winter. The gateway collects sensor data from each IAQD in turn at an interval of two minutes, and transmits data to server in the cloud via GPRS/4G. The authorized users can access cloud platform via mobile apps or the Web browser. Monitoring data analysis results reveal that the maximum PM2.5 concentrations is 10 times than usual during cooking period, and CO2 concentrations with the door closed will rise to 2500 ppm compared to the door opened at night, which threatens the health of occupants.