A bidirectional optical scanner assisting in mammography is revealed. The optical scanner that calculates functional images obtained by diffuse optical tomography, used in combination with a mammography machine can reduce the number of mammograms taken and the dose exposure. The bidirectional optical scanner includes a first compression plate, a first optical detection module, a second optical detection module and a second compression plate. The same test position of the tested breast can be detected twice in different directions by the first and the second optical detection modules. No matter where the tumor is located, the tumor can be detected. Besides structural images provided by the mammography machine, functional tomographic images of the breast are obtained by the bidirectional optical scanner. Thus diagnostic accuracy in the detection of breast cancer is improved.