Abstract: When fishing the string fish in sand producer, string milling length should be optimized and a downhole string dynamic analysis model is established to calculate the length. By using the dynamic analysis model, string friction and hook load during operation were calculated and the results provided theoretical backups for the choosing of engineering parameters. It is proved that the calculating model is simple and practical, and the complicated troubles during fishing operation can be minimized by using it; furthermore, it can improve operation efficiency and cut down the operation cost.