This paper develops a low-cost double-layer uncooled thermistor infrared detector based on standard 0.5μm CMOS technology and micromachining proces ses.The double-layer infrared detector employs a hidden-bridge-leg micro-bridge structure with a 55μm×55μm bridge deck,two 7.5μm width hidden bridge legs and an aluminum thermistor.The micro-bridge structure is im plemented by the surface sacrificial layer technology without any additional lithography or thin film de position procedure.The sacrificial layers include three materials: aluminum,tungsten and polysilicon.The double- layer infrared detector has a size of 65μm×65μm and a fill factor of 71.6%.The temperature coefficient of resista nce (TCR) of the aluminum thermistor is about 0.419%/K in a muffle furnace whose temperature varies from 10℃ to 100℃.The thermal conductance of the double-layer infrared detector is calculated as 1.96×10－5 W/K by measuring responses to different heating currents from 0.3mA to 4.8mA.The fabricated infrared detecto r is irradiated by an infrared laser which is modulated by a mechanical chopper in a frequency range of 10-1000Hz.Measurements show that the thermal time constant is 1.14ms and the thermal mass is 2.23×10－8 J/K.The respo nsivity of the infrared detector is about 2.54×10⁴V/W at 10Hz and the calculated detectivity is 1.6×10^8cmHz1/ 2/W.