Locating information source accurately is important for controlling its diffusion on the social network. In previous studies, a feasible way is locating the source using process information collected by the observers. Thus, the accuracy rate is closely related to the observer positions. In this paper, an optimal deployment method for observer positions is proposed. Considering the information diffusion process for single source, it firstly analyzes the relationship between the accuracy rate for locating a specified source and the positions of observers. Based on the relationship, it finds a key factor which is related to the accuracy rate of locating any source. It then suggests a method to deploy the observer positions based on r-coverage rate. It chooses the r-coverage rate of the observers as the objective function to implement the r-coverage rate first observer selection algorithm. The proposed method is tested on model and real networks respectively. Results show that the proposed method is effective. The observer deployment method is significant in controlling internet rumors and computer virus.