Benzothiophene is an under-explored monomer for free radical polymerization. Here we report the preparation of poly(benzothiophene-alt-maleic anhydride) (PBTMA) microspheres with high yield by self-stabilized precipitation polymerization. The chemical structure of the PBTMA copolymer was characterized by ¹H-NMR, ¹³C-NMR, FT-IR and MALDI-TOF MS analyses. The effects of various reaction parameters, including the reaction medium, the monomer concentration, the initiator concentration and the reaction time, on the morphology, particle size and molecular weight (M_n) of the resultant PBTMA microspheres were investigated systematically. The experimental results demonstrated that the PBTMA copolymer was an alternating copolymer, whose composition was independent of the monomer composition. Uniform PBTMA microspheres with the diameter in the range of 400 nm to 3000 nm and M_n up to 12 000 g mol⁻¹ were easily prepared. PBTMA had good solubility in common polar organic solvents and possessed much higher thermal stability than other maleic anhydride copolymers. PBTMA also showed interesting fluorescence properties both in the solid state and in solution. This research has laid the foundation for the further development of PBTMA copolymers as a new material.