Free spectral range (FSR) of reflection spectrum of micro-ring resonator restricts the improvement in user capacity of the optical code division multiple access (OCDMA) system using micro-ring resonator array. Vernier effects can expand FSR of cascaded optical microring resonator. Based on vernier effect, a novel two-dimensional coherent optical en/decoder using serially coupled dumbbell micro-ring resonator is proposed in this paper. The theoretical model of optical transmission for series dumbbell-shaped microring resonators is established. The relation between the suppression of pseudo-modes in optical reflection spectrum and the coupling coefficient is analyzed in detail. The effects of coupling coefficient, processing error and chip rate on the performance of series dumbbell microring resonator optical en/decoder are studied. The en/decoding simulation experiments are carried out on a series dumbbell-shaped micro-ring resonator with radius of 40 μm-30 μm-40 μm respectively. The results show that comparing with the traditional series micro-ring resonator with radius of 40 μm-40 μm-40 μm respectively, the FSR value of dumbbell microcavity is increased by 4 times and the user capacity can increase exponentially. Meanwhile, the ratio of autocorrelation peak to maximum wing (P/W) and the cross-correlation ratio (P/C) are increased by about 33% and 8%, respectively.