The rapid and real-time analytical system based on photometrical methodology of a homemade holographic flat field concave grating integrated microspectrometer was established,which was applied to detect the remnant formaldehyde in the intermediate of ranitidine synthesis.Based on the chromogenic reaction of formaldehyde with MBTH and NH4Fe(SO4)2 in acid medium,the methodology of formaldehyde detection was developed.Furthermore,factors affecting the chromogenic reaction were optimized in details.Under the optimized conditions,a good linear relationship in the range of 0.04-0.20 μg·mL-1 (r=0.993 8) was obtained and the lowest detection limit achieved 0.04 μg·mL^-1.In order to demonstrate the methodology and reliability of the microspectrometer analytical system,comparative experiments were carried on among the homemade microspectrometer,SHIMADZU UV-2550 spectrophotometer and U.S.A.Ocean Optic HR2000 Microspectrometer.The F-test was employed to check the reliability and efficiency of the proposed methodology.No significant difference was shown among them.The experimental results showed that the set up analytical system could satisfy the needs of quick and real-time detection in practical applications for experimental analysis,industrial monitoring and medicinal diagnosis.