Node self-positioning is one of the core technologies in wireless sensor networks and is the premise of many applications. It is significant to achieve highly efficient and reliable node localization for event observation, target tracking and efficient routing. The DV-Hop localization algorithm which is one of the Range-Free algorithms is researched deeply. Firstly, algorithmic process, error sources, and the amount of communication and computing for Traditional DV-Hop algorithm are analyzed. For low positioning accuracy in Conventional DV-Hop localization algorithm, an improved DV-Hop localization algorithm based on estimate of average hop distance, Min-Max and position correction is brought forward. The simulation results show that the improved algorithm can improve the positioning accuracy more effectively without increasing hardware consumption in nodes comparing with conventional DV-Hop algorithm. So it is a practical and effective node localization method in wireless sensor networks.