Abstract: Three categories of interplanetary disturbances have been chosen from the period 1978--1982 when solar activity was high. They are:(i)shock wave;(ii)strong magnetic field disturbance;(iii)strong magnetic field disturbance behind shock wave. The cosmic ray variations produced by them have been studied statistically.The following results are obtained:(1)cosmic ray decrease resulted from the strong magnetic field disturbance is very obvious when there is a shock wave before it,but the cosmic ray decrease is relatively small when only shock wave or strong magnetic disturbance exists;(2)the shock wave which indicates the speed discontinuity is a vital factor resulting the Forbush decrease;(3)the speed discontinuity may take a role of trigger in the process of the cosmic ray modulation produced by the strong magnetic field disturbance.