A wearable cardiac monitoring device comprises a processor； a electrocardiogram (ECG) signal collecting unit； a photoelectric signal collecting unit； and a power source configured to provide power to the processor, the ECG signal collecting unit and the photoelectric signal collecting unit simultaneously； wherein the processor determines whether the current mode is at a ECG collecting mode or a photoelectric collecting mode； wherein the ECG signal collecting unit collects user's ECG signals in the ECG collecting mode, and the photoelectric signal collecting unit collects photoelectric signals of the user's measured part under the illumination of light.
