On the basis of the semi-classical closed orbit theory, the effect of rectangular cavity on electron photo-detachment from a hydrogen negative ion was studied. Using the law of reflection, the electrons motion of closed orbit associated with photo-detachment cross section in the rectangular cavity was analyzed, and the formula of photo-detachment cross section in the system was deduced. Under the action of linear polarization light, the influence of the size of the cavity on cross section was studied, and the results were compared with that of the quantum mechanics method which MA Zhi-Jun used. It is shown that, the existence and the size of rectangular cavity induces significant oscillation in the photo-detachment cross section owing to the interference effects of the electron wave traveling along closed orbits and source function. The oscillatory structure in the photo-detachment cross section sensitively depends on the size of rectangular cavity. When the laser is linearly polarized in the x-axis or y-axis, quantum mechanics results are in line with the classical results. But when the laser is linearly polarized in the z-axis, rectangular cavity is not function on cross section along z-axis in actual situation, cross section is with the case of no field by semi-classical method, while the volatility is still exist in cross section by quantum mechanics method, which shows that the semi-classical method results are more accurate in this system. The results will be useful in understanding the photo-detachment of negative ions or electron transport in a micro-cavity.