This paper analyses and modifies some problems in original definition and algorithm of Frequency Hopping Processing Gain,and some concepts are clarified.It is favourable to expanding the applicable field,defining the technical indexes and checking the designed equipments.Based on which,the relationship between Frequency Hopping Processing Gain and the anti-jamming ability of the system is further discussed.And a concept of the threshold for anti-obstruct-jamming in common frequency hopping system is put forward yet.