This paper describes a method for laser beam alignment using plane mirror, which can be used in the watt/joule balance laser beam alignment. The mirror surface is calibrated by a vertical axis standard platform. An electrical level meter is used as readout unit and rotated on the platform. The plane mirror which is also on the rotation platform is then adjusted horizontal using laser and PSD readout. The experiment indicates that the vertical axis standard has the uncertainty of about 28μrad The incline of the plane mirror is estimated to about 3μrad.