Objective To investigate the dynamic change of platelets in patients with acute-on-chronic liver failure (ACLF) , the association of platelet count and its change with patients' prognosis, and the cause of thrombocytopenia, since severe thrombocytopenia is often seen in patients with liver failure.Methods A total of 54 patients with ACLF who were hospitalized in Beijing You' an Hospital, Capital Medical University, from September 2014 to September 2016 were enrolled, and their clinical data were collected.An automatic blood analyzer was used to measure platelet count, and ELISA was used to measure the level of thrombopoietin (TPO) .The t-test was used for comparison of normally distributed continuous data between two groups, and the Mann-Whitney U test was used for comparison of non-normally distributed continuous data between two groups; the chi-square test was used for comparison of categorical data between groups.The univariate and multivariate logistic regression analyses were used for prognostic parameters, and the receiver operating characteristic (ROC) curve was used to analyze the value of the change in platelet count in the diagnosis of liver failure.Results A total of 18 ACLF patients died before discharge, resulting in a mortality rate of 33.3%.These patients had a mean platelet count of 80.0 × 109/L [range (36.0-334.0) ×109/L], which was significantly lower than the normal value.The death group had a significantly greater reduction in platelet count than the survival group [ (-43.4 ± 58.9) × 109/L vs (-11.5 ± 29.1) × 109/L, t =-2.827, P = 0.041].With -27.5 × 109/L as the cut-off value of the change in platelet count, there was a significant difference in mortality rate between the platelet count change >-27.5 × 109/L group and the platelet count change ≤-27.5 × 109/L group (22.2% vs 72.2%, χ2= 12.623, P < 0.01) .The univariate analysis showed that platelet change, prothrombin time activity, and fasting blood glucose were influencing factors for patients' prognosis, and the multivariate analysis showed that platelet change was an independent influencing factor for the prognosis of ACLF patients, with an area under the ROC curve of0.743.The plasma level of TPO was measured for 24 patients, among whom 8 died before discharge, resulting in a mortality rate of 33.3%.There was no significant difference in the level of TPO between the survival group and the death group (91.8 ± 39.8 pg/ml vs 93.3 ± 46.5 pg/ml, t = 0, P = 0.938) .Conclusion Baseline platelet count in ACLF patients is significantly lower than the normal value.The reduction in platelet count is an independent risk factor for death before discharge in ACLF patients.Thrombocytopenia may not be associated with the reduction in TPO.