In order to eliminate distortions of stereo image,the paraxial magnification of stereoscopic camera and reproduction system need to be studied.The paraxial magnification formula was rederived and discussed.The new formula adds a coefficient to represent the paraxial magnification of observation and reproduction which is in the ratio of the image distance to the distance between the eyes and screen.The formula accords with the result of verifying experiment.It is important for further study on the distortions elimination of stereoscopic camera and display system.