In order to improve the safe and reliable embedded system,software development design phase needs to consider security as soon as possible.Therefore,this paper presented a threat model for embedded system that analyzed the possible threats in embedded system.For evaluating the scale of thread,the method established the threat model in the form of trees and calculated the thread value of tree's nodes from bottom to top.It used the thread of meter billing information in smart meters as an example to illustrate the process of building the threat model and calculating the thread value of each node that verifying the practicability and effectiveness of this method.