Background The second-trimester maternal serum screening in twin pregnancy is still controversial, as the serum marker levels in twins are not as clear as those in singletons. This study aimed to evaluate the relationship between the levels of the second-trimester maternal serum free p-human chorionic gonadotropin (free β-HCG) and a-fetoprotein (AFP) in normal twin and singleton pregnancies and to estimate feasible analysis methods for utilizing these markers in second trimester screening for twin pregnancy. Methods On the basis of a prospective population-based study of second-trimester maternal serum screening, the concentrations of maternal serum AFP and free β-HCG of 195 normal twin pregnancy and 26 512 singleton controls at gestational weeks 15 to 20 were measured by time-resolved fluoroimmunoassay in one laboratory. The levels of markers were compared between the twins and singletons using weight-correction and gestational age-specific model. Results According to the research protocol, 95 communities were randomly sampled, which covered the whole Jiangsu province, the east of China. A total of 26 803 pregnant women (98%), from the target population accepted prenatal screening for maternal serum AFP, β-HCG detection, and all babies were followed up for at least six months. There were 197 (0.73%) twin pregnancies, of which one case had fetal trisomy 18, and one case with fetal anencephaly. The others were normal twin pregnancy. From a total enrollment of 26 803 women participants, 26 512 women with normal singleton pregnancies were selected as the model controls. The other 291 pregnancies, including trisomy 21, neural tube defect (NTD), trisomy 18, and other fetal abnormalities, were excluded. No significant differences were found in the medians of gestational age-specific maternal serum free β-hCG and AFP in normal twin pregnancy comparing with twice those in model controls with the exception of the medians for free β-hCG during the 16th gestational week (P=0.012). Conclusion The weight-correction and gestational age-specific levels of Chinese Han population maternal serum free P-hCG and AFP in normal twins were twice the levels as those in the singleton controls during the 17-19 gestational weeks.