On-line vibration monitoring system can timely find the vibration in the early signs of wind turbine fault, reveal the cause, degree, site and development trend of the failure, etc.. It is advantageous for the wind machine maintenance. At the same time ,It can avoid and reduce the occurrence of major accidents, save cost .The paper first introduced the development background and present situation at home and abroad of condition monitoring system, and then pointed out the necessity of vibration monitoring in wind turbine . The composition and the structure of on-line monitoring system are indicated, and the distribution of measurement point on the spot is discussed in accordance with the vibration features of the wind turbine. Finally ,this paper puts forward the problems of online monitoring system to be solved in the future.