The paper is about the emission of fluorine from an experimental boiler of 0.25 MW when burning black liquor coal-water slurry (CWS). The result is compared with CWS. And the analysis of effect factors is also carried out. Experimental research show that, the content of fluorine in CWS, which was less than 30 mg/kg, was relatively low. The emission ratio and concentration of fluorine were 70% - 90% and 2.0 - 2.6 mg/m3 respectively when burning common CWS; while the data of those when combusting black liquor CWS were 45% - 80% and 1.7 - 3.0 mg/m3 respectively. The hearth temperature and the element of CWS were two factors of emission fluorine when burning CWS.