Abstract: The propagation constant and the optical field distribution of the multimode interferometer were calculated precisely, and the results were compared with the results of the analytical approximate calculation. From the results, it is founded that there are two reasons for the decrease of output optical power under weak constrain. Firstly, the quantity of guided modes supported by the waveguide is small, which causes the poor self-imaging quality and great loss of the highfrequency components, forms the big dispersed spots, and results in the loss of the output coupling. Secondly, the partial energy is taken away by the direct radiation of the high-frequency radiating mode.