The invention belongs to the technical field of sludge flow measurement and in particular relates to an easily precipitated water quality open channel flow measuring instrument and a measurement method thereof. The open channel flow can be accurately measured on line in real line and is not influenced by the sludge precipitation thickness, and the problem that the open channel flow is inaccurate to measure is solved. In order to achieve the aim, according to the technical scheme disclosed by the invention, the measuring instrument comprises a photoelectric type sludge thickness measuring part, a liquid impact force and flow velocity measuring part, a handheld terminal and a data processing mainboard, wherein the photoelectric type sludge thickness measuring part is connected to a signal output port of a mainboard shell via a communication line by virtue of a transmitting board; the liquid impact force and flow velocity measuring part is connected to a force measuring signal port of the mainboard shell by virtue of a force measuring signal line; the handheld terminal is connected to an RS485 communication port of the mainboard shell by virtue of an RS485 data communication line; and the data processing mainboard is respectively connected with the signal output port, a signal receiving port, the force measuring signal port and the RS485 communication port.