An optical method is proposed to extract the displacement vector of domain wall (DW) sidewise motion from ferroelectric domain configuration pattern on a tetragonal Mn:Fe:KTN crystal. The ferroelectric domain configuration evolution under an external electric field is observed in situ using optical microscope imaging system, and the relative displacement of DW sideways motion between two adjacent electric fields is extracted. By the combination of the new domain length vector extracted, the ferroelectric domain displacement vector as a function of field strength can be obtained; what is more, the hysteresis loop can be achieved during field cycling. The method provides an access to directly manipulating DW motion and exploring the properties of ferroelectric materials from ferroelectric domain configuration pattern.