Objective Platelets are increasingly considered to play an important role in inflammation and are being regarded as a putative bridge linking mental diseases and inflammatory response. Platelet-associated haematological parameters including mean platelet volume (MPV), platelet distribution width (PDW), plateletcrit (PCT), systemic immune-inflammation index (SII), platelet to lymphocyte ratio (PLR), platelet to albumin ratio (PAR) and red blood cell distribution width (RDW) to platelet ratio (RPR), have been recently investigated as simple, easily available, and inexpensive inflammatory markers. In this study, we aimed is to use large-scale clinical data to study platelet parameters in patients with affective disorders, to further investigate the predictive power of platelet parameters for major depressive disorder (MDD) and bipolar disorder (BD). Methods The retrospective, naturalistic, cross-sectional study analysed the data of 14,007 Chinese affective disorder patients, including 4,801 patients with first-episode MDD, 4,098 patients with recurrent MDD, 3,444 patients with BD manic episodes and 1,664 patients with BD depressive episodes. Meanwhile, 6,847 healthy subjects were served as the control group. The differences in the MPV, PDW, PCT, SII, PLR, PAR, RPR and albumin among different groups were compared, and the contributing factors for the occurrence of MDD or BD were analysed. Results There were significant differences in MPV, PDW, PCT, SII, PLR, RPR and albumin values among the study groups. In the subjects, patients experiencing BD manic episodes had the highest mean values of MPV and SII, patients experiencing BD depressive episodes had the lowest mean values of platelet counts and PAR, and patients with MDD had the highest mean values of PLR and RDW. The levels of MPV, PDW and albumin were independently correlated with MDD and BD, and they are important predictors for differentiating patients with MDD or BD from healthy controls. Conclusions Our study demonstrated that different affective disorders have unique platelet parameter variation patterns, highlighting the role of platelet parameters and systemic inflammation in the pathophysiology of MDD and BD.