Abstract: The law of transmission of Citrus Huanglungbin by psylla and the distribution of pathegen in psylla have been studied since 1984. The results show that Huanglungbin agent can be transmitted by a single adult after feeding on the diseased citrus plant. It will infect tested seedlings at a high efficiency of 80%.The pathogen cannot be transmitted by a single adult after a 1-3-hour acquisition feeding on the diseased citrus plant,nor by a nymph of the 1st to 3rd instar. The infected adult is unable to transmit pathogen through its egg. But the pathoren can be transmitted by a single adult after an acquisition feeding of more than 5 hours on the diseased citrus plant and also by a nymph of the 4th to 5th instar. A freshly emerged adult from an infected old nymph is able to transmit the pathogen to the tested citrus seedlings and the pathogen in an old nymph can be transmitted to an adult emerging from the nymph, It is able to transmit the pathogen by striding instar. Since psyliid needs fresh shoots to lay eggs on, and the shoots of dissased citrus plant grow in a disorderly way, the Huanglunbin diseased tree in an orchaned is not only a tree of the pathogen source, but also tree of the vector source. On condition of psyliid transmitting pathogen by striding instar, psyliid growing on and spreading from diseased plant is the epidemic source of Huanglungbin disease in orchards. It is an important law on transmission of pathogen by psyliid.