The mechanism on frequency limit control (FLC) improving frequency stability is well analyzed, characteristics between reverse frequency differential model and integrated negative feedback model are studied, the operation of FLC on Yunnan asynchronous interconnection network is introduced with the proposal of FLC parameter optimization on Xinsong-Dongfang UHVDC Project, and finally the FLC malfunction and operation suggestion are put forward. The conclusions are: 1) FLC can well improve frequency stability of Yunnan Power Grid in case of generators tripping, heavy load disturbance and HVDC accident. 2) It causes neither constant transformer tap changing nor constant AC filter action. 3) Xinsong-Dongfang UHVDC FLC system owns a desired DC power modulation after FLC parameter optimization. 4) Reverse frequency differential model gains an advantage over integrated negative feedback model and deserves wide use.