Some important nodes in power grid have serious impacts on cascading failures. Choosing reactance of transmission line as the weight parameter， a weighted power network model based on complex network theory is established. The weighted network agglomeration is defined. The important node evaluation indicator is proposed based on the weighted power network model. With this method， both network topology of the power grid and distribution of electrical characteristics are considered. The time-domain simulation on EPRI 36 bus system shows the effectiveness of the method.