Abstract: In this paper,experimental measurement error will produce when the spectroscopical plane and compensator is not parallel to each other and two plates are not perpendicular to the guide rail.In addition,it will also cause the difficulty of the operation of the experiment.And we put forward the spectroscopical plane and compensator to be an L type to solve the problem of two board type deviation.After many experiments,the short axis ratio of the interference pattern is increased from 0.60 to 0.80,and the relative error of the measured optical wavelength is increased from 2.4 to 1.7.