BeiDou Navigation Satellite System (BDS) has got the ability of real‐time navigation and high accuracy positioning services for the Asian‐Pacific regions by the end of 2012 with the constellation of 5GEOs+5IGSOs+4MEOs.Currently,although BDS can provide positioning service better than 10 meters,,there remains a problem that the positions calculated with B1 and B2 pseudo‐range ionosphere‐free combination (BC) are a little worse in accuracy than those obtained from B1 pseudo‐range only.This phenomena can be partly explaned with the observing errors (white noise and multi‐path),and the accuracy of ionosphere model.Besides,the coupled error for BDS satellite clock‐TGD would be another important factor.Here we analyze this coupled error and its effect on BDS real‐time positioning based on BDS precise clock and BDS satellites DCB provided by GNSS Research Center (GRC),Wuhan University.12 days’ BDS observations from BeiDou experimental tracking stations (BETS) have been processed and investigated with the precise products by GRC and broadcast ephemeris.Results indicate that the coupled error between BDS broadcast satellite clock and TGD of BC is 5.04ns,which is about 3 times larger than that of B1 (1.79ns).And it is a system bias that results BC positioning accuracy worse than that of B1.In additional,by analyzing BDS observations’ multi‐path noise,it shows the multi‐path noise is one of the facts leading to BDS BC positioning accuracy a litter worse than B1 positioning accuracy.We also find that the receiver noise is the main reason affecting BDS velocity accuracy.