A blackout occurred in East China power grid in 2009 and its evolution process are introduced. In accordance with three stages of failure development， status of equipment concerned and protection information are analyzed respectively. Analysis shows that the fault is a rare and complex one， but improper blocking between 35 kV bus coupler auto-switching（BCAS） and bus differential protection（BDP） is a contribution factor to escalation of the fault. Based on dissection of the BCAS protection and BDP principles in this substation， the mutual blocking relationship between these two protections is analyzed， and hidden failures in the design is pointed out. Eventually， the advices and precaution measures are presented.