The adjustment of the position of the optical trap is of great importance for its practical application. In this paper, the imaging regularity of the laser waist through an inverted microscope is analysed. A method is advanced to observe the position of the waist and to measure its deviation (d) from the objective plane with an object plate. Furthermore, based on the transformation of the Gaussian beam, the position of the laser trap can be easily adjusted by regulating the collimating lens for the semiconductor-laser (the sourse of the laser teap). This meyhod is simple and adapts to various biological specimens.