The ex-situ catalytic fast pyrolysis (CFP) of blue-green algae (BGA) and its nitrogenous model compounds over HZSM-5 was conducted in a microfluidized bed reactor (MFBR) combined with a U-shape fixed bed reactor (U-FBR). A single photoionization mass spectrometer (SPI-MS) was used for on-line studying the dynamics of volatiles formation with the biomass-to-catalyst ratio (BCR). In the CFP of BGA, six different groups of products were differentiated by statistical analysis of SPI mass spectra, which were obtained with the cumulative BCRs. The relative concentration of each group has its own evolution with catalyst deactivation, indicating the different formation pathways of six groups. In combination with results from the CFP of six model compounds, the reaction pathways of nitrogenous compounds in the CFP of BGA over HZSM-5 were proposed. The benzyl nitrile could be fully converted to hydrocarbons while this is hard to achieve N-cyclic compounds (particularly the pyridines). Besides, most of nitrogenous compounds could be interconvertible in the CFP process. Transmethylation is the favorite way for catalytic conversion. In addition, the CFP of BGA and its N-cyclic compounds would lead to the fast coking of HZSM-5, especially the hard coke.