The invention discloses a biomass fuel and semi-coke mixed burning system and method. Aiming at the problems of low calorific value of biomass gasification products, difficulty of semi-coke ignition steady burning, high discharge of NOx and the like, the biomass fuel and semi-coke mixed burning method is adopted, gasification is performed on biomass fuel through a gasification furnace, flammable gaseous products are fed into an ignition area and a reburning area which are arranged inside a furnace cavity, meanwhile, solid residues are fed into a main burning area for mixed burning with semi-coke, the problem of difficulty of semi-coke ignition steady burning is solved, and by means of layering mixed burning inside the furnace, a reducing atmosphere area is formed at the upper portion of the main burning area, so that burning optimization is realized, the NOx discharge quantity is reduced at the same time, and meanwhile, part of hot air inside an air preheater is fed into the gasification furnace and a two-stage cyclone separator is adopted, so that the air fan investment is reduced, the energy utilization rate is increased, and the gasification furnace efficiency is improved.