Regarding the electrostatic field in Mollenstedt biprism as the fieid of the cylindrical capacitor and considering the phase shift of free-electron caused by the cylindrical field, the difference of optical path length and the spacings of the interference fringes on the plane for observation were calculated. The angle of deviation for free-electrons in the electrostatic biprism and the width of the interference region formulars were obtained. And the coherent condition was discussed.