Abstract: In fringe reflection technique, the phase ambiguous error can influence the measurement result directly. The existing error reduction methods to compensate this error are inflexible and can only reduce a part of this error. An accurate phase-slope analytical model without phase ambiguous problem is deduced. Based on the simulation of the deduced model, it is found that phase ambiguous error can close to zero when the system light rays are both perpendicular to the reference plane and the display screen. A system based on the co-axial optical path is proposed to achieve this system setup. In our experiment, with the increasing of the test surface height from 0 to 3 mm, the proposed co-axial system is proved to have the phase ambiguous error always close to zero. Meanwhile, the phase ambiguous error in the tradition system increases from 0 to 1.5 rad when the test surface height rises from 0 to 3 mm.