Abstract: In the construction of synthetic seismogram,the cor- relation between synthetic seismogram and practical seis- mic trace is considered to be the only criterion to deter- mine the superiority and inferiority of synthetic seismo- gram.In order to improve the correlation,unlimited ex- tension and compression are often conducted.Starting from average velocity and fluctuation phenomenon oc- curred in high correlation of synthetic seismogram,by means of interval velocity easy to identify,abnormal high -velocity layer and abnormal low-velocity layer are found to be correspondent with the fluctuation phenome- non.Therefore,the process and cause of the fluctuation phenomenon are analyzed.In the end,a method to avoid the fluctuation in mean velocity curve and its application are provided,which lays a good foundation for fine inter- pretation of structure and fine investigation of reservoir.