The invention provides a capacity detecting method and system for an elevator emergency power supply and the elevator emergency power supply. The elevator emergency power supply includes a charging circuit, a storage battery, a control and signal acquisition module, a discharge circuit, a circuit switching module and a signal output module; the control and signal acquisition module acquires current identification information of the storage battery, inquires target discharge current and target discharge time corresponding to the current identification information, and controls the circuit switching module to communicate the discharge circuit with the storage battery, according to the target discharge current and the target discharge time, the discharge circuit is controlled to partially discharge electricity to the storage battery, after the partial discharge is over, the current temperature value and the current open-circuit voltage value of the storage battery are measured, and according to a battery capacity attenuation curve corresponding to the current open-circuit voltage, the current temperature value and the current identification information, the capacity value of the storage battery is obtained; the obtained capacity value is output by the signal output module. By means of the method, the system and the elevator emergency power supply, the manpower cost can be saved, and the method and the system can be applied to the capacity detection of different types of storage batteries.