The invention provides a coal pyrolysis chemical chain gasification coupling process based on low-rank coal gradient utilization. Coal pyrolysis is performed to obtain coal tar, coal gas and semicoke; the semicoke is applied to a chemical chain gasification process aiming at synthesis gas preparation. According to the chemical chain gasification process, Fe2O3 is used as an oxygen carrier to react with the semicoke in a gasification reactor for obtaining the synthesis gas with CO and H2 as major ingredients and ferriferous oxides with FeO/Fe as major ingredients; the ferriferous oxides enter a coal pyrolysis reactor as a coal pyrolysis solid thermal carrier to generate corresponding pyrolysis products; separated FeO/Fe and FeO/Fe in the gasification reactor re-enter an oxygen carrier regeneration reactor to have an oxidation-reduction reaction with the air; an obtained high-valence ferric oxide Fe2O3 releases heat at the same time. The heat transfer of each operation unit can be realized through the material circulation of the ferric oxides in each reactor; the utilization efficiency of the coal is improved through the organic coupling of the coal pyrolysis and the chemical chain gasification; the energy consumption is reduced; the low-rank coal gradient utilization is realized.