The present disclosure relates to a method and a system of quantitative intravascular optical coherence tomography. The method includes: acquiring and graying an intravascular optical coherence tomography cross-sectional image, and converting it into a polar coordinate view; calculating a confocal function value and a sensitivity function value according to parameters of the imaging system; establishing a model of an intensity of a backscattered signal being attenuated depending on the detection depth, according to the confocal function value and the sensitivity function value to obtain a theoretical value of the backscattered signal; determining a measured value of the backscattered signal; constructing an error function according to the theoretical value and the corresponding measured value; minimizing the error function to determine a light attenuation coefficient corresponding to each polar coordinate; and determining a spatial distribution diagram of the light attenuation coefficient according to the light attenuation coefficient corresponding to each polar coordinate.