Abstract: Studying the key points of multi-frequency navigation system positioning,generally discussing the characteristics of multi-frequency linear combination observations,comparing the means of multi-frequency ambiguity single epoch fixed which are TCAR,CIA and LAMBDA,discussing the mode of GPS / GALILEO multi-frequency combined positioning,and simulating the GPS/GALILEO multi-frequency data.The processing results of simulating data shows that the means of TCAR/CIA/LAMBDA can fix ambiguity single epoch in short baselines and the LAMBDA get the highest success.GPS / GALILEO combined positioning will increase the baseline accuracy to 2 cm.