Mixing blast furnace slag (BFS) in coal gasification coarse slag (CGCS) based geopolymer improves its early mechanical properties. In this paper, CGCS and BFS were used as raw materials to prepare geopolymer, and the effect of different BFS content on the early mechanical properties and microstructure of geopolymer was studied. The microstructure of activated CGCS-BFS based geopolymer was characterized by X-ray diffraction, mercury intrusion porosimeter, scanning electron microscope and Fourier transformed infrared spectroscopy. The results show that the compressive strength of geopolymer increases gradually when the BFS content increases. When the BFS content reaches 40% (mass fraction), the 28 d compressive strength of sample is as high as 531 MPa. According to the microscopic analysis, a large amount of calcium/sodium silicate aluminate hydrate (C(N)-A-S-H) gel is formed on the surface of the geopolymer, which makes the microstructure of geopolymer more denser, and thus improves the mechanical properties of geopolymer.