Abstract: In this paper,on the basis of a compressible magnetohydrodynamic model,the dynamics of the cometary plasma tail with a super-Alfvenic flow in the lobes arestudied numerically. The results show that the streaming tearing instability is excited by the sheared plasma flow which exists between the plasma sheet and the lobes.It causes magnetic reconnection and forms the magnetic islands and plasmoids with high plasma density in the plasma sheet of cometary tail.Furthermore the effect of a local driving force delivered by the solar wind on magnetic reconnection in the plasma tail of comets is also simulated.But its characteristic time is larger than that of the streaming tearing mode.It is suggested that some of knots and disconnection events observed in the plasma tail of comets may be mainly associated with the streaming tearing instability produced by the sheared plasma flow in cometary tail.