Abstract: By using a four-layer model, the stabilities of ME type spiral sector transition region in the interplanetary magnetic field are discussed. The results show that (1) three kinds of large-scale waves may be excited in MB type transition region, and for all of them, there exists a low-frequency cut-off; (2) the growth rates of these three kinds of instabilities all increases as k increases, but among them only the growth rate of model A has a maximum and a minimum; (3) whenk is parallel or antiparallel to the velocity V_q of solar wind in the sector regions, the excitation of waves is easier; when the angle between k and V_q become larger, the cut-off wave length moves to the shot end; when k is perpendicular to V_q, the waves can not be excited; (4) when the angle between k and V_q is 75°, the wave with a wave length of 5×10⁴ km and phase velocity 340 km/s may be excited, this is in agreement with teh observations at the earth's magnetopause obtained from Voyager 1. Hence, we deduce that the waves in the spiral sector transition region may be a perturbation source causing the K-H instability in the magnetopause.