Abstract: Aim at the problem that vertical distance cannot be estimated correctly which caused by reflecting signal's phase invertion, a simple method which can automatically detects phase inversion, corrects it and make a uniform image is putted forward.This paper makes a brief description of the arithmetic of the signal processing for SPR. It also discusses how the reflecting signal's phase is affected by the reflecting surface, when the uniform planewave touch the perfect metal or dielectric surface. It proves the validity of the arithmetic, when the real data is processed. So SPR's ability of detection is improved greatly when there is phase invertion.