Abstract: Objective  To explore the changes of coagulation function indexes in children with pneumonia of different severity(common pneumonia, critical pneumonia and extremely critical pneumonia), so as to provide a valuable reference for clinical evaluation of the disease. 
Methods  A total of 320 children with common pneumonia, critical pneumonia and extremely critical pneumonia who were admitted to our hospital were enrolled in this study. Among them, there were 182 cases of common pneumonia, 91 cases of critical pneumonia and 47 cases of extremely critical pneumonia. Blood was drawn from all children within 24 h after admission, and the coagulation function, plasma D-dimer(D-D), platelet and other indicators of each group were analyzed and compared. In addition, the correlation of these indicators with pediatric critical illness score(PCIS)was analyzed. 
Results  Prothrombin time(PT), activated partial thromboplastin time(APTT), fibrinogen(FIB), fibrinogen degradation products(FDP) and D-D were higher in critical pheumonia group and extremely critical group than in common pneumonia group(P＜0.05), and FDP and D-D in extremely critical pneumonia group were higher than those in critical pneumonia group(P＜0.05). No significant difference was found in PT, APTT and FIB in extremely critical pneumonia group and critical pneumonia group(P＞0.05). PLT count in critical pneumonia group and extremely critical pneumonia group was lower than that in common pneumonia group, which was lower in extremely critical pneumonia group than in critical pneumonia group(P＜0.05). There was no significant difference in mean platelet volume(MPV) and platelet distribution width(PDW) among three groups(P＞0.05). PLT count, FIB, FDP and D-D were significantly correlated with PCIS. PLT was significantly and positively correlated with the PCIS, while FIB, FDP and D-D were significantly and negatively correlated with the PCIS(P＜0.05). 
Conclusion  There are significant differences in the coagulation function indexes among three types of pneumonia of different severity. Among them, PLT, FIB, FDP, and D-D can effectively judge the severity of children with critical pneumonia, which is of great clinical significance for clinicians to choose a reasonable treatment scheme, and improve the cure rate and the prognosis of children with critical pneumonia. 

Key words: pneumonia, children, blood coagulation function