尝试使用merge，并且只使用df1中的列 ID

merge(df1[,"ID",drop=F], df2, by.x="ID", by.y="Name")
  ID Score
1  A    70
2  A    70
3  B    54
4  B    54
5  B    54
6  B    54
7  C    99
8  C    99

使用dplyr

library(dplyr)

left_join(df1[,"ID",drop=F], df2, c("ID" = "Name"))
  ID Score
1  A    70
2  A    70
3  B    54
4  B    54
5  B    54
6  B    54
7  C    99
8  C    99

数据

df1 <- structure(list(ID = c("A", "A", "B", "B", "B", "B", "C", "C"), 
    Score = c(NA, NA, NA, NA, NA, NA, NA, NA)), class = "data.frame", row.names = c(NA, 
-8L))

df2 <- structure(list(Name = c("A", "B", "C"), Score = c(70L, 54L, 99L
)), class = "data.frame", row.names = c(NA, -3L))