The invention discloses a relative installing and calibrating method for a star sensor. The method comprises the following steps that: S1, the star sensor is connected to a star sensor error calibrating system; S2, a three-axis angle of the star sensor changes to drive a Fresnel prism in the star sensor error calibrating system to generate the same three-dimensional rotating angle, and a metering unit in the star sensor error calibrating system calculates to obtain the three-dimensional rotating angle according to a linear displacement change amount of an image point in an image which is acquired by an imaging CCD (charge coupled device); S3, the metering unit in the star sensor error calibrating system introduces the obtained three-dimensional rotating angle into an installing error compensating matrix to obtain a relative installing error of the star sensor. The method has the advantages that the three-dimensional rotating angle of the start sensor is converted into an optical image point movement linear amount by utilizing an optical auto-collimating angle measurement mechanism, and then, the change value of the rotating three-dimensional angle of an object is calculated through the movement amount of the image point, so that the relative installing error of the star sensor is calibrated.