We propose to measure the vector velocity of the flow field based on bidirectional scanning in optical coherence tomography (OCT). Transverse velocity was determined by dynamic light scattering OCT (DLS-OCT) under four introduced transverse synthetic speeds, while axial velocity was obtained by Doppler OCT (DOCT). The flow fields within two straight channels and an improperly welded Y-junction were measured. In vivo measurement on a rabbit was further conducted. Without resorting to system modification and requiring only four scan biases, the proposed method is advantageous in measurement of the vector velocities of a complex flow field.