Abstract: Starting from the variance-covariance of multi-frequency carrier phase combination's equation,every observation error expression is deduced one by one via the propagation of errors,and the impact of positioning precision with the changing of observation error analyzed.Using only one carrier phase combination to high precision positioning,a method to get the optimal combination is put forward.The result indicates that this method's arithmetic is simple and the precision is excellent,so it can beused for long baseline quick positioning for different precision request.