The disclosure provides an underwater detector, a groundwater flow rate and flow direction measuring instrument and a method thereof. The underwater detector comprises a sealed casing; an infrared sensing module is arranged in the sealed casing and can rotate around the main axis along a horizontal plane at a fixed angular velocity; and an electronic compass is arranged on the infrared sensing module. The bottom of the sealed casing is connected with an indicator by a rotating shaft; the indicator can rotate freely on the horizontal plane along the rotating shaft under the action of water flowand includes a head and a tail provided with a light source; the light source is used for emitting a light beam vertically and upwardly; and the light beam can be received by the infrared sensing module. The infrared sensing module is used for triggering the electronic compass to record a real-time direction relative to the main axis when receiving the light beam; and a processor module carries out processing to obtain a water flow direction. A spring is connected between the head and the tail. The infrared sensing module is also used for transmitting a time of receiving the light beam to theprocessor module directly; and the processor module is used for calculating the water flow rate.