In order to know the stress development in the film during growth and to study the origin and mechanism of the stress much deeply, an instrument based on light beam deflection for in-situ stress measurement in optical films by using two light beams is presented. An improved system is used to get the variation of the distance between the two beams. It is proved that the precision of the instrument is 2.2% after several times of measurement. This precision can satisfy the stress measurement of the thin film. The stress curve of SiO2 during growth is presented. As a result, the instrument represents an easy, stable, and high precision technique. It supplies an effective method for measuring the intrinsic stress.