An in situ decoupling analytical method was proposed and a multistage in situ reaction analyzer based on a micro fluidized bed (MFB-MIRA) was developed for in situ and on-line analysis of multistage gas-solid reaction processes. The responses of the pipe system to various gas concentration waves were analyzed. The in situ decoupled combustion properties of three coals were investigated using a two-stage reaction procedure involving devolatilization in Ar and combustion in O-2/Ar. The Peclet number of the pipe flow was as large as 87, indicating approximate plug flow in the MFB-MIRA. A concentration correction model was built to simulate the real gas release process in the micro fluidized-bed reactor. The measurement distortions in the char combustion concentration curves were found negligible, which proved the reliability of using the measured results directly to derive the carbon conversion processes. The MFB-MIRA gives a good mass balance. The combustion reaction orders of the three in situ chars in O-3 were all about 0.9 during the main reaction period and decreased monotonously with increasing carbon conversion after the maximum rate. Calculations of heat and mass transfer during char combustion showed that isothermal reaction conditions were guaranteed and combustion during carbon conversions of 0.3-0.8 were kinetically controlled. This study proves that the MFB-MIRA is suitable for rapid gas solid reactions.