Objective To investigate the diagnostic value of prenatal ultrasonic quantitative indexes in predicting the prognosis of fetal renal pelvis widening.Methods The fetuses found to have renal pelvis dilated detected by prenatal ultrasound in our hospital from July 2017 to March 2019 were collected, and they were divided into the second trimester group, third trimester group and subgroups. The difference of anteroposterior diameter of renal pelvis（APD）and the renal parenchymal thinning ratio between pathological and physiologic renal pelvis widening in the second trimester and third trimester group was compared.Based on the ultrasound diagnosis results of 2 years after birth,with persistent renal pelvis dilation and renal calices bluntness as the criteria for the pathological pelvis widening.Receiver operating characteristic（ROC）curves were drawn to analyze and compare the diagnostic efficacy of APD and renal parenchymal thinning in the second and third trimesters for pathological renal pelvis widening.The kidneys of renal pelvis dilatation from the second trimester to the third trimester were collected,and the efficacy of the second trimester combined with the third trimester APD in diagnosing pathological renal pelvis widening was calculate. Results A total of 1512 kidneys were included, of which 829 were in the second trimester group, including 23（2.8%）of pathological renal pelvis widening; there were 390 kidneys in the third trimester group, including pathological pelvis widening in 25（6.4%）.There were statistically significant difference in the APD and renal parenchymal thinning ratio between pathological and physiological pelvis widening in the two groups（both P＜0.001）. ROC curve analysis showed that the area under the curve of pathological renal pelvis widening by APD in the second and third trimesters were 0.974 and 0.982, respectively, which was better than the area under the curve（0.897 and 0.875）of renal parenchyma thinning in the same period, and the differences were statistically significant（P=0.0361、0.0036）.The 0.8 cm and 1.1 cm were used as the cut-off values, respectively, and the corresponding sensitivity was 95.7% and 100%, the specificity was 92.8% and 88.8%.The APD indexes are applied in combination with kidneys that exhibit pyelectasis from the second trimester to the third trimester, and the diagnostic specificity can reach to 98.5%. Conclusion Ultrasound measurement of APD in the second and third trimesters of pregnancy can effectively predict the prognosis of fetal renal pelvis widening, and its diagnostic efficacy is better than the renal parenchymal thickness measured in the same period.