This paper discusses the Hopf bifurcation control problems in a multi-parameter space of power systems by taking power loads as parameters. In power systems， the power load's variation and the control decision-making and implementation process， such as capacitor switching， are implemented at different time scales. The former is a quite slow process， and the latter can be implemented in a short time. So， in general， the loads cannot change remarkably during the control decision-making process. Based on the above fact， this paper proposes an on-line method to control Hopf bifurcations in power systems. The principle of the proposed method is: When the equilibrium（or operating） point is closing to the Hopf bifurcation surface， the approximate normal vector of the bifurcation surface corresponding to the current equilibrium point is calculated directly， and then the controlling parameters are regulated according to the approximate normal vector to avoid a Hopf bifurcation in a multi-machine power system. At last， the validity of the method is demonstrated on a numerical example of a 14-bus power system.