An adjustable ultrasonic gas flow measurement device includes a gas pipe, a transmitter, a receiver and a micro-processing module. The gas pipe is filled with a gas under test. The transmitter and the receiver are respectively installed in two ends of the gas pipe for transmitting and receiving ultrasonic signals inside the gas pipe. The micro-processing module is electrically connected to the transmitter and the receiver for generating and sending a first ultrasonic signal to the transmitter for transmitting, and comparing the phase shift of a second ultrasonic signal and a third ultrasonic signal to output a phase comparison result, wherein the transmitted first ultrasonic signal passing through the gas pipe was received by the receiver as the second ultrasonic signal, and the third ultrasonic signal is generated by shifting the first ultrasonic signal with a predetermined phase difference.