In order to achieve ocean water level monitoring for effective tsunami warning, an allfiber interferometric water level sensing system is proposed and demonstrated. According to the large range and fast response of ocean water level variation, an interference structure based on doublecollimator alignment and a linear frequencychirp based phase demodulation method are adopted, achieving the ocean water level measurement with large range, high precision and fast response. The experimental results show that the measurable range of the water level sensing system is 0~109.14 m, the measurement accuracy can reach ±1 cm, with the response time of 0.01 s. The proposed system is demonstrated to be suitable for ocean water level monitoring and tsunami warning.