The invention discloses a six-DOF displacement measuring method based on a planar capacitor. The six-DOF displacement measuring method consists of a movable polar plate and a fixed polar plate; a square capacitance electrode array is distributed on the fixed polar plate; eight rectangular sensing capacitance electrodes are arranged on the movable polar plate and are divided into four groups; two electrodes in each group have a 1/4 cycle position difference in the measurement direction and the output signal has a phase difference of 90 degrees; when the movable polar plate generates the displacement in XY plane relatively to the fixed polar plate, a sum-to-product equation group is combined with the output signal of the eight capacitors, thus solving the linear displacement signal along X axis and Y axis and the rotation angle signal around Z axis; when the movable polar plate generates an inclined displacement, by combining the capacitance solution equation group, the linear displacement signal along Z axis and the rotation angle signal along X axis and Y axis are solved. By solving two equation groups, the method realizes the six-DOF measurement on three linear displacements along X axis, Y axis and Z axis and the three linear rotation angle displacements around X axis, Y axis and Z axis.