In a fluid queueing model, maintaining buffers and avoiding system congestion are the focus of attention. For the purpose, this paper combines a threshold regulation with a variable service rate, and studies such a fluid model with the threshold regulation and the variable service rate in a fully observable case. Firstly, the equilibrium balking strategies of fluid are discussed. Secondly, the social benefit per unit time of the model, and the influence of fluid level and threshold on social benefits per unit time are analyzed by numerical examples. Finally, the total revenue of admission fee per unit time is obtained, and the change of the total revenue of admission fee per unit time with the threshold is discussed. The results of this paper will provide theoretical basis for the application of fluid queueing model.