The invention discloses a self-contained pressure compensating system and a pressure monitoring method thereof. The self-contained pressure compensating system comprises an on/off control device, an oil supply device, a pressure compensating device and a power unit, wherein the pressure compensating device can be used for detecting the pressure change in a self inner cavity in real time; the power unit is associated with the pressure compensating device; the pressure compensating device supplies oil to the power unit; according to the detection result of the pressure compensating device, the on/off control device triggers the oil supply device to supply the oil to the pressure compensating device if the pressure in the inner cavity is less than a predetermined first threshold, and triggers the oil supply device to stop supplying the oil to the pressure compensating device if the pressure in the inner cavity is more than a predetermined second threshold. Therefore, the pressure in the inner cavity of the pressure compensating device can be detected in real time; the automatic oil supply can be realized; the pressure compensation can be effectively provided for the power unit.