The invention discloses an underground flow tester and an underground flow testing method. The underground flow tester is installed on a section through which injected water flows. The underground flow tester comprises a housing, wherein a pipeline through which the injected water flows is arranged inside the housing, a generator through which the injected water can be formed to swirl fluid is arranged at the front end of the pipeline, a guide cone which is used for guiding the swirl fluid is arranged at the rear of the generator, a piezoelectric crystal probe which is used for converting pulsating impulse signals of the swirl fluid to electrical pulse signals is arranged at the rear of the guide cone, the piezoelectric crystal probe is provided with an arc-shaped receiving surface which is used for receiving the pulsating impulse signals, and an extending cavity which is used for enabling the swirl fluid to form fluid which flows smoothly is arranged at the rear of the piezoelectric crystal probe in the pipeline. By the aid of the method and the tester, the underground flow can be monitored in real time, and the power consumption is low during monitoring.