Abstract: Objective To investigate the relationship between serum soluble fms-like tyrosine kinase receptor 1 (sFlt-1), complements C1q and C5a levels and perinatal outcomes in patients with severe preeclampsia at different onset times. Methods A total of 60 pregnant women with severe preeclampsia were selected as a research group, including 30 early onset cases and 30 late onset cases of severe preeclampsia. Blood samples were collected on admission without drug treatment. Meanwhile, 30 normal pregnant women were selected as a control group. Blood samples were collected at 34 weeks pregnant and before full-term delivery. Perinatal outcomes were recorded in all groups. The levels of serum sFlt-1 and C5a in pregnant women in each group were detected by ELISA. The level of serum C1q was determined by immunity transmission turbidity. Results Compared with the control group, the research group produced remarkably increased levels of serum sFlt-1 and C5a and decreased levels of serum C1q (P<0.05). The levels of serum sFlt-1 and C5a increased but the levels of serum C1q decreased in severe preeclampsia pregnant women with early onset, compared with those with late onset (P<0.05). The levels of serum sFlt-1 and C5a were positively correlated with the incidence of fetal adverse outcomes in pregnant women with severe preeclampsia, while the levels of serum C1q were negatively correlated with the incidence of fetal adverse outcomes (P<0.05). Conclusions For pregnant women with severe preeclampsia, their serum sFlt-1 and C5 are highly expressed but serum C1q are lowly expressed. Their changes are obviously associated with preeclampsia severity and fetal adverse outcomes. It indicate that abnormal activation of the complement system may be associated with the onset of severe preeclampsia.