The invention discloses a method for measuring a wind tunnel density field based on video and a sub-pixel technology, which includes: S1, carrying out image acquisition on a density field before and during a wind tunnel test respectively to obtain corresponding static density field measurement images and dynamic density field measurement images; S2, locating centers of circles in the dynamic density field measurement images based on a sub-pixel positioning technology; and S3, calculating an optical refractive index with position change of coordinates of the centers of circles in the dynamic density field measurement images and coordinates of the centers of circles in the static density field measurement images so as to calculate and obtain a wind tunnel density field. The method provided by the invention is able to calculate the wind tunnel density field by comparing the position change of the centers of circles in background images during and before the test based on video measurement. Compared with the prior art, the method has the advantages of being simpler and more convenient, high precision and fast measuring speed.