Abstract: Abstract: In order to understand the thermal decomposition mechanism of seaweed polysaccharide and the formation mechanism of main products from the micro perspective, two main seaweed polysaccharides molecules, enteromorpha clathrate sulfated polysaccharides and sargassum fusiforme fucoidan were selected. Through Py-GC/MS and TG-MS experiments, the formation mechanism of products was analyzed. Py-GC/MS results showed that enteromorpha clathrate polysaccharides pyrolysis forms furans mainly, with 5-methyl-furfural as the typical product; while the products of sargassum fusiforme pyrolysis are mainly polysaccharide acid esters. The polymers builder of hyperchem and the semi-empirical method were used to build and optimize the molecular model of sulfated polysaccharides and fusiforme fucoidan. Then the characteristic parameters of molecules were obtained. Based on amber force field, the pyrolysis processes of the seaweed polysaccharide model compounds were simulated with the molecular dynamic method at 297-1200 K and periodic boundary conditions. The simulation results showed that the pyrolysis of lignin model compound can be divided into three stages: the low temperature stage (below 500 K), the intermediate temperature stage (500-800 K) and the high temperature decomposition stage (above 800 K). It was found that the reaction stage concentrated mainly in the intermediate temperature stage. The simulation results showed that hydroxide radical was produced at about 400 K. And when the temperature was increased to about 500 K, polysaccharides molecular monomers were formed with the break of glucoside bond. At the same time, pyranoid ring was also opened with the formation of various kinds of molecular fragments. The experimental results were basically in agreement with those of pyrolysis simulation. Besides, the formation mechanisms of H2O, CO2, and SO2 during the pyrolysis of seaweed polysaccharides were analyzed combined with TG-MS gas release mass spectrometry. The GC-MS were adopted to analyze the composition of bio-oil which was obtained through fast pyrolysis. By comparing the experimental and simulational value, the influence of those pyrolysis gases content on the distribution of pyrolysis product and the reaction pathways was also analyzed. The results of TG-MS verify that the release of H2O from seaweed polysaccharide occurs in the temperature range of 250-350 ℃. CO2 mass spectrometry shows that there are two major precipitation peaks in the pyrolysis process, one at low temperature (200 ℃), and the other at high temperature (800℃). SO2 mass spectra shows that there are two precipitation peaks in the range of 280-350 and 650-800 ℃. In the process of the seaweed polysaccharide pyrolysis, the maximum precipitation peaks of pyrolysis gases correspond to the DTG peaks under fixed heating rate. The three main light gases (H2O, CO2 and SO2) show a bimodal characteristic. The precipitation of H2O is mainly caused by the alkyl and carboxyl group fracturation. The main reason of the CO2 generation is related to the decarboxylation reaction occurs in pyrolysis process. CO2 precipitation is mainly due to the C-O-C fractured in low temperature range, and the main reason of the CO2 generation is the biaryl ether decomposition in high temperature range. SO2 precipitation is mainly due to the reaction of elimination of sulphate in low temperature range, and the main reason of the SO2 generation is the sulphate decomposition in high temperature range.