The current-voltage properties of CdS/CdTe heterojunction are discussed, then, the limit efficiency of CdS/CdTe heterojunction solar cells are investigated under AM1.5 illumination. Analysis results show that the efficiency of solar cells can be achieved to 27% at idealized limit conditions without any lost, which is in agreement with previously reference result. At the same time, the realistic efficiency of CdS/CdTe heterojunction solar cells is about 23%, when some effect of material parameters are considered. From the considerable progress of CdS/CdTe solar cells during recent years, theoretical efficiency of 23% can be expected in the near future.