Based on introducing signals in space of Galileo system and under the condition that the ambiguity is kept as integers, this paper defines the phase combination observations of the 4 Galileo and GPS L2 carriers. The relevant error impact is analyzed and the method for defining the ranges of searching combination coefficients is expatiated. Accordingly two sets of applied interfrequency combination observations with typical characters are also listed. Finally, some valuable inter-frequency combinations are given.