This paper presents a transmissive rapid scanning optical delay line (RSOD) for balance detection based optical coherence tomography (OCT). This transmissive RSOD,consisting of a reflective blazed grating,a Fourier lens,a fast-scanning galvo-mirror,a right-angle prism and a mirror,possess functions of phase modulation,axial scanning and dispersion compensation in OCT just as a reflective RSOD. The developed transmissive RSOD is implemented in OCT for balance detection. Comparison study indicates that improved quality of imaging is archived by the transmissive RSOD based OCT,in contrast to that by a conventional reflective RSOD based OCT.