Nitrous acid is one of the major sources of atmospheric cleaner OH radical, and its impact on the atmospheric oxidation potential is the hotspot of the atmospheric physical chemistry research in recent years. However, due to the low concentration of nitrous acid in the atmosphere, and its reactivity, solubility and photolysis, real-time accurate measurement of nitrous acid is very difficult. The results of trace HONO detection using wavelength modulation technology based on the 1255 cm-1 room-temperature continuous wave quantum cascade laser are reported, the minimum detectable HONO concentration of 6.95 μg/m3 which is detected by direct absorption method reaches up to 0.84 μg/m3 in 1 s integrated time. The optimal integrated time of the system can reach 100 s by Allan variance analysis, the corresponding time detection limit is 0.34 μg/m3 . HONO in the air of the laboratory is successfully detected using this system, which proves that the system can satisfy the requirements of detection limit for the actual atmospheric measurement.