A large Forbush decrease (Fd) has been recorded by Moscow neutron monitoring station on 17th July 2017. An attempt has been made to understand the probable causes which have contributed for the occurrence of the large Fd. During the period of investigation extreme solar and interplanetary events i.e. sun spot numbers, disturbance storm time (Dst) index and Ap index has been investigated. The Dst index is suddenly increased during the event period indicating high geomagnetic activity during event time. The rise in sun spot numbers earlier the onset of forbush decrease reveals that it should have some connection with the occurrence of solar flares which further effects the cosmic ray variation. Probably this decrease is a result of high activity on the sun. The forbush decrease under investigation is probably due to the solar flare generated shock wave magnetic perturbation.