The gas density of the supersonic molecular beam (SMB) is a crucial parameter for the fueling or diagnostic process in the tokamak experiments. Using the microphone, one improved method of gas density measurement is proposed, which can greatly improve the measurement capacity by about 3 orders of magnitude by studying the pulsed signal characteristic of the microphone when it is pushed by the SMB. The gas density of the SMB is measured within the axial range of 20-2000 mm, and the axial central density at 2000 mm is about 100 times less than that at 20 mm. It is also found that the radial density distribution follows the Gaussian function in both free expansion (where the SMB can expand freely without any influence from the vacuum chamber) and restricted expansion (where the expansion of the SMB is restricted inside the flight tube of the vacuum system). And the axial central density decreases with the axial distance, which follows the inverse square law in the free expansion, but it deviates from this law in the restricted expansion.