The invention provides a transient quantity based protection starting method and device. The protection starting method comprises the following steps: a transient current in a protected system is sampled; a differential current of each phase according to data obtained via sampling operation is obtained, and all differential currents between phases are calculated according to the differential current of each phase; according to an amount of change of all the differential currents between the phases, that relay protection needs to be started is determined when the amount of change of any differential current between the phases is greater than a set value. Via the transient quantity based protection starting method and device provided in the invention, a transient quantity in a protected system can be solved in a differential manner, whether the relay protection needs to be started is determined according to the amount of change of all the differential currents between the phases, interference caused by noise of a hardware sampling loop can be eliminated, and reliability of relay protection of an electric power system can be improved.