Objective@#To observe the effect of chorioamnionitis on placental microvessel and platelet metabolism in premature and the correlation between them.@*Methods@#With clinical randomized controlled trial (RCT), the cases were matched by 11 according to gestational ages and divided into 2 groups according to the placental pathology results: chorioamnionitis group and control group, 32 cases in each group.Dates were obtained for preterm infants (gestational age<37 weeks) admitted to the Department of Neonatology at Guangdong Women and Children Hospital, born between June and December 2016.The platelet parameter [platelet count (PLT), mean platelet volume (MPV), platelet distribution width (PDW), plateletcrit (PCT)], birth weight, thrombocytopenia, hemorrhage complication, miscrovascular density (MVD) in placenta, platelet activating factors (CD62p, CD63) and thrombopoietin (TPO) in preterm infants were recorded and compared.@*Results@#In chorioamnionitis group, the infant birth weight[(1.90±0.41) kg vs.(2.31±0.62) kg] and the PLT in 72 hours [<24 h (197.97±63.43)×109/L vs.(266.34±69.92)×109/L; 24-72 h (202.28±29.70)×109/L vs.(256.38±69.96)×109/L] were significantly lower compared with the control group, and the differences were statistically significant(all P<0.05). The incidence of early thrombocytopenia(37.50% vs.9.38%), intracranial hemorrhage(40.62% vs.15.63%), MPV [(8.73±0.89) fL vs.(8.27±0.64) fL] and PDW[(59.46±5.90)% vs.(55.20±5.37)%] in 24 hours were significantly higher in chorioamnionitis group, and the differences were statistically significant (all P<0.05). The placental MVD in chorioamnionitis group significantly decreased[(9.08±1.35)% vs.(12.89±1.36)%, P<0.05 ]. The level of CD62p, CD63 and TPO in umbilical cord blood were significantly higher in chorioamnionitis group[(25.37±5.20)% vs.(10.35±2.94)%, (9.49±1.58)% vs.(4.04±1.21)%, (271.08±197.22) μg/L vs.(141.87±78.10) μg/L, all P<0.05]. The placental MVD was positively associated with PLT (r=0.74, P<0.05) and negatively associated with CD62p, CD63 and TPO among infants with chorioamnionitis (r=-0.64, -0.44, -0.44, all P<0.05).@*Conclusions@#The chorioamnionitis may decrease the MVD in placenta and activate platelet in fetal circulation, damaged placental microvessel may activate platelet further.