The flow separation of very thick wind turbine airfoils always causes dynamic performance of the airfoils. The unstable aerodynamic performance of the 45% thick airfoil, CAS-W1-450, was studied in a wind tunnel at Re = 1.0 x 10<^>6. Two types of aerodynamic accessories, i.e., vortex generators (VGs) and M-type trailing-edge device (M-plate) were used to change the surface condition. And then, the effects of the accessories on the unsteady separation were studied. The results show that the thick airfoil has serious three-dimensional boundary-layer separation at large angles of attack. Bifurcation appears typically near stall condition because the time-averaged flow mode cannot maintain a single state and the pressure signals obviously jump/drop from one value to another. As a result, the testing data near stall and at some of other conditions scatters a lot. Measurements of VGs and M-plate show that VGs can delay or eliminate both stall and bifurcation on the airfoil, but M-plate has less effect on the bifurcation because perturbations downstream have less effect on upstream flow. As for flow conditions at very large angles of attack, it is found that the time-averaged aerodynamic performance varies less and is repeatable, although separation is strongly unsteady at very large angles of attack. (C) 2018 Elsevier Ltd. All rights reserved.