Abstract: This paper introduces Galileo system in brief,then presents a comprehensive study of the inter-frequency combinations of 4 Galileo carrier phase observations.This paper discusses some error impacts of Galileo combination observations,expatiates the method for defining the ranges of searching combination coefficients in detail,and accordingly lists two sets of applied inter-frequency combination observations with typical characters:widelane combinations and reduced-iono combinations.As a result,some valuable inter-frequency combinations are found.