Williams, 34, from Llanfaethlu, Holyhead, Anglesey, suffered a serious head injury and was airlifted to Selly Oak Hospital, where she died.
The accident was at fence eight on the novice course. The horse, Shannandoah, a 14-year-old dark bay gelding owned by Jo-Anne with 28 British Eventing points, number 228 in Open Novice Section E, was uninjured.
Alex Fox, Chairman of British Eventing, said: "On behalf of the whole sport, I would like to extend our deepest sympathy to Jo-Anne's family. The thoughts and prayers of eventing people all over the country are with them at this sad time."
British Eventing officials are investigating the accident.
• Later the same day another horsewoman was airlifted to Selly Oak with a severe spinal injury after a fall at a livery stable in Stourport-on-Severn.