The resonant cavity length of microcavity influences the light emitting characteristics of microcavity organic light emitting device (MOLED) directly. According to the related calculation formula of microcavity device, when the lengths of microcavity are lambda/2 and lambda, the authors use transfer matrix method to simulate and compare with the functions of composite light emitting EL when exciton is in different positions of microcavity. The authors found that when the length of microcavity is lambda/2, the peaks of luminous spectrum are all at the 520 nm, and the width of half-peaks are all 17 nm. The peak intensity and integral intensity are biggest when exciton is in the central area of microcavity. When the length of microcavity is lambda and exciton is at different positions of microcavity, the peaks of luminous spectrum are all at the 520 nm of designed center wavelength, and the widths of half-peaks are all 12 nm. The peak intensity and integral intensity are smallest when exciton is in the central area of microcavity. After analyzing, The authors found that the light emitting characteristics is best when the exciton is at the maximum position of the electric field. This is because the electric field's intensities in the microcavity with two kinds of lengths are distributed differently. It illustrates that one should distinguish different resonant cavity length and exciton at the maximum position of the electric field within microcavity if one wants to create an efficient MOLED.