In this paper, we fabricated thin-film saturable absorber by mixing 2D material tungsten diselenide (WSe2) and polymer polyvinyl alcohol (PVA). The WSe2-PVA saturable absorber (SA) was incorporated in an erbium-doped polarization maintaining (PM) fiber laser, and stable Q-switched operation was realized. An all PM cavity can reduce environmental perturbation on the cavity birefringence. Based on this PM cavity, we investigated the stability of WSe2-PVA SA in the Q-switched operation. It is found that when the heat accumulation reached a certain threshold in the SA, the polymer substrate firstly melted and more heat accumulateed quickly. As a result, WSe2 was damaged, showing a significant increase of SA loss.