Radar echo extrapolation is an effective way to select the operation time in rocket weather modification. The radar echo extrapolation method of TREC (Tracking Radar Echoes by Correlation) is described. Based on the echo shifting and moving speeds, echoes are extrapolated linearly within the range of the rocket shot, then calculating the time that the echoes arrives at the rocket shot to provide reference for the selection of operation time. The test indicates that the extrapolated time for echoes to reach the rocket shot is in consistent with the actual echoes.