Disclosed is a downhole flow measuring while drilling device capable of measuring downhole annular flow, annular pressure and annular temperature and monitoring early overflow while drilling. The measuring device measures downhole annular flow and flow change through an ultrasonic wave sensor mounted at a short section of a drill collar and measures annular pressure and annular temperature through a pressure and temperature sensor mounted inside an annular fluid guiding device, a power source and a circuit board are mounted inside an exploring pipe, physical signals, such as annular flow, pressure and temperature, are converted into electric signals through the sensors, the measuring device records, stores and uploads data through the circuit board, and the measuring device is connected with a MWD (measurement while drilling) through an MWD joint to timely upload actually-measured data to the ground. The device is simple in structure, low in manufacturing cost and long in service life, and working conditions such as downhole overflow can be judged in real time by comprehensively analyzing and processing timely-uploaded downhole data and related data on the ground to achieve the objective of early overflow monitoring, so that safety of well control of well drilling is effectively improved.