The development and improvement of the global navigation satellite system (GNSS) provides an important basis for multi-constellation's fusion positioning, navigation, and timing, and it also provides more favorable external conditions for the receiver's autonomous integrity monitoring. Firstly, the protection level of GNSS single system and multi-system combination under single-frequency and dual-frequency conditions were calculated respectively through the constellation simulation. Then, This paper analyzed the basic distribution of protection levels under different frequencies and different combinations of GNSS worldwide. Finally, the availability of receiver autonomous integrity monitoring (RAIM) in different aviation approaching phases under different frequencies and combinations was discussed. The calculation results show that the single system under single-frequency or dual-frequency combination cannot meet the requirements of the approach phase of localizer performance with vertical guidance 200ft (LPV-200).Nevertheless, double-system under dual-frequency can meet the requirement.The triple-system under dual-frequency combination can meet the requirements of approach with vertical guidance II (APV-II), but cannot meet the requirements of precision approaching. But the improvement of four-system combination is not obvious compared with that of triple-system combination.