Precise fiber positioning is crucial to a wide field, multi-fiber spectroscopic survey like LAMOST. Nowadays, most position error measurements are based on CCD photographic and imaging processing techniques. Those methods only work for measuring errors orthogonal to the telescope optical axis, while there also lies errors parallel to the telescope optical axis, like defocusing, and error caused by the existing deviation angle between optical axes of a fiber and the telescope. Directly measuring two latter types of position errors is difficult for individual fiber, especially during observation. Possible sources of fiber position errors are discussed in brief for LAMOST. By constructing a model of magnitude loss due to the fiber position error for a point source, we propose an indirect method to calculate the total and systematic position errors for each individual fiber from spectra data. Restrictions and applications of this method are also discussed.