This paper aims to develop a detection system that can accurately measure indoor formaldehyde emissions. For this purpose, the existing formaldehyde detection methods were reviewed in details, and the gas sensor method was selected for our research. Then, a formaldehyde concentration detection system was set up based on the STC12C5A, including a micro-controller unit (MCU) module, a sensor module, a liquid-crystal display (LCD) module and an alarm module. Through comprehensive tests on three different sites, it is proved that the system can capture the exact formaldehyde concentration indoor and display the results on the LCD; an alarm could be released when the concentration exceeds the preset threshold. Overall, the system is stable, reliable and easy to operate in various indoor environments.